Abstract: In order to investigate the main off-flavor compounds of fermented soymilk, headspace-solid phase microextraction (HS-SPME) combined with gas chromatograph-mass spectrometer (GC-MS) was used to identify the flavor compounds of fermented soymilk/milk mixture, and sensory evaluation was conducted for these fermented mixture samples. 44 flavor compounds were isolated and identified, 20 compounds in soymilk were determined identical to those in milk. aldehydes, alcohols and furans of fermented soymilk decreased with the increasing milk content, the off- flavor of fermented soymilk may come from aldehydes, alcohols and furans. As milk content increased, the concentrations of 21 compounds in fermented soymilk/milk mixture decreased. Through the analysis of flavor characteristics, threshold, relative content, trends, and sensory evaluation of fermented soymilk/milk mixture, the results showed that the most important compounds contributed to the off- flavor of fermented mixture were 1-octen-3-ol, hexanol, 2-heptenal and 2,4-decadienal.