#include    <stdio.h>
#include    <stdlib.h>
int main()
{
    FILE * fp;
    char word[20];
    char find[20];
    char ch;
    int len,i;
    printf("Please put in the word you want to look up:\n");
    gets(word);
    len=strlen(word);
    if((fp=fopen("5000_words.txt","r"))==NULL)
    {
        printf("Can not open the file !!!\n");
        exit(0);
    }
    fgets(find,len+1,fp);                     //求大佬指点把它改成只赋值每个换行符之后的len长度的字符给find字符数组
    while(ch!=EOF)
    {
        if(strcmp(find,word)==0)
        {
            while(ch!='\n')
            {
                ch=fgetc(fp);
                printf("%c",ch);
            }
            exit(0);
        }

        for(i=0;i<len;i++)
            ch=fgetc(fp);
        fgets(find,len+1,fp);
    }
    if(ch==EOF)
        printf("Not found！！\n");
    puts(find);
    fclose(fp);
    return 0;
}

已解决，谢谢