自己找到答案了，给与我有相同疑问的人参考吧，更详细的opt规定可以在这里找到：
（http://www.ice.gov/sevis/updates_postcompletion_opt.htm）
For students who are not on a STEM extension, this employment may include:
Paid employment. Students may work part-time (at least 20 hours per week
when on post-completion OPT) or full-time.
Multiple employers. Students may work for more than one employer, but all
employment must be related to the student’s degree program and for pre-
completion OPT cannot exceed the allowed per week cumulative hour
