#include <stdio.h>
#include<stdlib.h>

int main()
{
    struct student
    {
        int num;
        char name[20];
        float score;

    };
    struct student student1;
    struct student student2;
    scanf_s("%d,%s,%f", &student1.num,student1.name,sizeof(student1.name), &student1.score);
    scanf_s("%d,%s,%f", &student2.num, student2.name,sizeof(student2.name), &student2.score);
    printf("The higher score is:\n");

    if (student1.score > student2.score)
    printf("%d %s %6.2f\n", student1.num, student1.name, student1.score);
    else
    {
        if (student1.score < student2.score)
            printf("%d %s %6.2f\n", student2.num, student2.name, student2.score);
        else
        {
            printf("%d,%s%6.2f\n", student1.num, student1.name, student1.score);
            printf("%d,%s%6.2f\n", student2.num, student2.name, student2.score);
        }
    }
   
    system("pause");

}

要这样写
    scanf_s("%d,%f,%s", &student1.num, &student1.score, student1.name, sizeof(student1.name));
    scanf_s("%d,%f,%s", &student2.num, &student2.score, student2.name, sizeof(student2.name));

可是运行的结果还是不对，只是想打印出成绩较高的那一个的 num,name,score ,结果却都打印出来了。。

不会啊：
1,100,zhang
2,90,li
The higher score is:
1 zhang 100.00
请按任意键继续. . .

en,好了，谢谢。只是不知道为什么要把sizeof放到后面，放到中间不行吗，分给你了哈哈。

因为scanf读字符串时是以空格，回车或EOF来结束的，你用‘，’是无法区分字符串是否结束的，所以放在后面以回车来结束。
或者你改成这样：
    scanf_s("%d%s%f", &student1.num, student1.name, 20, &student1.score);
    scanf_s("%d%s%f", &student2.num, student2.name, 20, &student2.score);