//实现简单的学生成绩信息管理软件。
   // 学生信息包括：学号、姓名、4门课程的成绩（计算机，数学，英语，物理）。
   // 要实现的功能：学生信息的录入、修改、删除和查询。
#include<stdio.h>
int n;
struct student
    {
        int id;
        char name[20];
        float com,math,en,phy;
    }stu[20];
    struct student *a=&stu[0];
void record(struct student *p)
{
    int i;
    printf("Input the number of students.\n");
    scanf("%d",&n);
    printf("Input the information of each student.\n");
    for(i=0;i<n;i++)
        scanf("%d%s%f%f%f%f",&(p+i)->id,(p+i)->name,&(p+i)->com,&(p+i)->math,&(p+i)->en,&(p+i)->phy);
    getchar();
}
void correct(struct student *p)
{
    int i,b,flag=1;
    printf("Input the ID of a student which you want to correct.\n");
    scanf("%d",&b);
    for(i=0;i<n;i++)
       if(b==(p+i)->id)
       {
          scanf("%d%s%f%f%f%f",&(p+i)->id,(p+i)->name,&(p+i)->com,&(p+i)->math,&(p+i)->en,&(p+i)->phy);
          flag=0;
       }
       if(flag)
        printf("No such student.\n");
       getchar();
}
void Delete(struct student *p)
{
    int i,b,flag=1;
    printf("Input the ID of a student you want to delete.\n");   
    scanf("%d",&b);
    for(i=0;i<n;i++)
        if(b==(p+i)->id)
        {
            (p+i)->id='\0';
            flag=0;
        }
        if(flag)
        printf("No such student.\n");
        getchar();
}
void search(struct student *p)
{
    int i,b,flag=1;
    printf("Input the ID of a student you want to search.\n");   
    scanf("%d",&b);
    for(i=0;i<n;i++)
        if(b==(p+i)->id)
        {  
            printf("%-4d%-10s%-6.1f%-6.1f%-6.1f%-6.1f\n",(p+i)->id,(p+i)->name,(p+i)->com,(p+i)->math,(p+i)->en,(p+i)->phy);
            flag=0;
        }
        if(flag)
        printf("No such student.\n");
        getchar();
}
int main()
{
    char c;
    while(1)
    {
        printf("A record  B correct  C delete  D search  E end\n");
        scanf("%c",&c);
        if(c=='A')       record(a);
        else if(c=='B')  correct(a);
        else if(c=='C')  Delete(a);
        else if(c=='D')  search(a);
        else if(c=='E')  break;
    }
    return 0;
}
//第二次输入学生数据会把以前的数据覆盖；希望帮我改一下，谢谢！




   

你程序写的就是这个功能，每次带入的都是数组的起始地址，自然就覆盖数据了
你要保留数据，那么就返回记录的位置或者返回指针，也可以选择改函数参数，具体实现方法看你自己想法了

如上楼所说，程序改动如下：
#include<stdio.h>
int n=0;
struct student
    {
        int id;
        char name[20];
        float com;
        float math;
        float en;
        float phy;
    }stu[20];
    struct student *a=&stu[0];
struct student *record(struct student *p)
{
    int i;
    printf("Input the number of students.\n");
    scanf("%d",&n);
    printf("Input the information of each student.\n");
    for(i=0;i<n;i++)
    scanf("%d%s%f%f%f%f",&(p+i)->id,(p+i)->name,&(p+i)->com,&(p+i)->math,&(p+i)->en,&(p+i)->phy);
    getchar();
    p=&stu[n];
    return p;
}
void correct(struct student *p)
{
    int i,b,flag=1;
    printf("Input the ID of a student which you want to correct.\n");
    scanf("%d",&b);
    for(i=0;i<n;i++)
       if(b==(p+i)->id)
       {
          scanf("%d%s%f%f%f%f",&(p+i)->id,(p+i)->name,&(p+i)->com,&(p+i)->math,&(p+i)->en,&(p+i)->phy);
          flag=0;
       }
       if(flag)
       printf("No such student.\n");
       getchar();
}
void Delete(struct student *p)
{
    int i,b,flag=1;
    printf("Input the ID of a student you want to delete.\n");   
    scanf("%d",&b);
    for(i=0;i<n;i++)
        if(b==(p+i)->id)
        {
            (p+i)->id=0;
            flag=0;
        }
        if(flag)
        printf("No such student.\n");
        getchar();
}
void search(struct student *p)
{
    int i,b,flag=1;
    printf("Input the ID of a student you want to search.\n");   
    scanf("%d",&b);
    for(i=0;i<n;i++)
        if(b==(p+i)->id)
        {  
            printf("%-4d%-10s%-6.1f%-6.1f%-6.1f%-6.1f\n",(p+i)->id,(p+i)->name,(p+i)->com,(p+i)->math,(p+i)->en,(p+i)->phy);
            flag=0;
        }
        if(flag)
        printf("No such student.\n");
        getchar();
}
int main()
{
    char c;
    struct student *p=&stu[0];
    while(1)
    {
        printf("A record  B correct  C delete  D search  E end\n");
        scanf("%c",&c);
        if(c=='A')       p=record(p);
        else if(c=='B')  correct(a);
        else if(c=='C')  Delete(a);
        else if(c=='D')  search(a);
        else if(c=='E')  break;
    }
    return 0;
}

将record函数修改一下：
student * record(struct student *p)
{
    int i;
    printf("Input the number of students.\n");
    scanf("%d",&n);
    printf("Input the information of each student.\n");
    for(i=0;i<n;i++)
        scanf("%d%s%f%f%f%f",&(p+i)->id,(p+i)->name,&(p+i)->com,&(p+i)->math,&(p+i)->en,&(p+i)->phy);
    return  p+i;
}
主函数修改一下：
int main()
{
    char c;
    student * st=stu;
    while(1)
    {
        printf("A record  B correct  C delete  D search  E end.\n");
        scanf("%c",&c);
        if(c=='A')       st=record(st);
        else if(c=='B')  correct(a);
        else if(c=='C')  Delete(a);
        else if(c=='D')  search(a);
        else if(c=='E')  break;
    }
    return 0;
}
原因就像楼主所说。

谢谢指导