do i need medical certficate for 6months of study visa in China?

I'm a little puzzled by your statement, you had already spent one year on student visa in China, do you mean you need to study in China for one year and a half? or do you want to study for more 6 months? If you need to apply for a F visa (F visa is issued to foreigner students who study for less than 6 months), a medical certificate is not required.