Hi, Xuanye:
It sounds like you have been talking with Dr Chen regarding research so if you two have talked about projects, you can select him as your research advisor. Your selection of a research advisor should be based on your interests and where they intersect the needs of a faculty member.
As part of your assistantship, you will be supporting the department in our needs and for that, Drs. Xie, Chen and I will talk about what we need you to do based on your background. We should have a better answer for you in the next week or so. You will be expected to support the department approximately 10 - 15 hours per week and then anything else will be on your research.
I hope this helps..