Visa Expires on May 2,2018

Hi, I have L, double entry with 30 days stay each duration and it will expire on May 2,2018. I have used my first entry and im planning to enter China again April 29. My question is, if I enter China before May 2 and stay for 30 days, is my visa still valid and do i need to get another one while in China so that I will not be charge for overstay?