Abstract: Soybean was chosen as material of this research, and the process of post- fermentation of fermenting soybean with Mucor was chosen as the researching object.solid phase micro-extraction and gas chromatography-mass spectrometry ( SPME-GC-MS) were used to determine the volatile substances of fermented soybean with low- temperature fermenting condition at different time.62 volatile compounds were identified, which could be classified as 9 categories including alkanes ( 2) , olefins ( 29) , aromatics ( 6) , alcohols ( 10) , aldehydes ( 1) , ketones ( 1) , acids ( 2) , esters ( 8) , ethers ( 3) .The results indicated that during the fermentation, the flavor compounds such as aromatics, aldehydes and ketones were increased, the contents of alcohols and acids compounds were reduced while esters compounds were increased, the contents of bad flavor 1- octen- 3- ol etc were reduced gradually.Fermentation in low temperature is beneficial to reduce the browning, promote the formation of flavor substances.