전공한 or 전공하는 - Present tense modifier

전공하는 is the right form. 전공한 is used for the past tense.

For example,

1. 나는 중국어를 전공한 학생이야.

This means that I finished studying Chinese.

2. 나는 중국어를 전공하는 학생이야.

This means that I am currently studying Chinese.

You mean 'a student who majors in Chinese'? -> 중국어를 전공한 학생 / 중국어를 전공하는 학생. Both are okay. -는 is an ending of a word that implies that an event or action is happening in the present.

So 중국어를 전공하는 학생 emphasizes more on studying Chinese right now as a major, while 중국어를 전공한 학생 suggests that the student completes the process to get the degree for Chinese as his major.