#include"stdio.h"
double GetReal(char *s);     /*用来实atof的函数*/
int ten_n(int);
void main()
{
    char s[]="  -3454e-3";
    double n;
    n=GetReal(s);
    printf("%f",n);
}
double GetReal(char *s)
{
    int i,i1,j,k,s1[100],s2[100];
    double n,n1,n2;
    char *p;
    p=s;
    while(*p==' ')
        p++;
    if(*p=='-')
    {
        p++;
        i=0;
        while(*p!='e'&&*p!='E'&&*p!='.')
        {
            s1[i++]=(int)(*(p++)-48);
            printf("%d",s1[i-1]);
        }
        printf("\n");
        if(*p=='e'||*p=='E')
        {
            p++;
            i1=0;
            if(*p=='-')
            {
                p++;
                do
                {
                    s2[i1++]=(int)(*(p++)-48);
                }while(*p!='\0');
                k=0;n1=0;
                for(j=i-1;j>=0;j--)
                    n1+=s1[k++]*ten_n(j);
                k=0;n2=0;
                for(j=i1-1;j>=0;j--)
                    n2+=s2[k++]*ten_n(j);
                printf("n2=%d\n",n2);    /疑问在这里:这里n2=3才对，怎么运算出来n2=0呢？/
                n2=-n2;
                n=n1+(double)ten_n(n2);
                return n;
            }
        }
    }
}
int ten_n(int n)
{
    int s=1,i;
    for(i=1;i<=n;i++)
        s*=10;
    return s;
}

printf("n2=%d\n",n2);    /*疑问在这里:这里n2=3才对，怎么运算出来n2=0呢？*/

n2被定义为double型，要加强制类型转化：
printf("n2=%d\n",(int)n2);
下面一句 n2=-n2; 要删去，因为你的 ten_n(int n) 函数中没有写n是负数的代码；
接下来一句应改为：

n=n1/(double)ten_n((int)n2);