Cowpea was fermented by kefir grains. The sensory score and total acid content were used as evaluation indicators. Single factor test and orthogonal test were used to optimize the fermentation process. And the changes of flavor substances before and after fermentation were analyzed by high performance liquid chromatography （HPLC） and gas chromatography-mass spectrometry （GC-MS）. The results showed that the best fermentation process conditions of cowpea by kefir grains were as follows: salt concentration 4%, kefir grains inoculation amount 1.5%, fermentation temperature 25 ℃, fermentation time 4 d. Under these conditions, the sensory score was 87 points, and the total acid content was 3.40 g/kg. A total of 72 volatile flavor substances were detected in cowpea fermented by kefir grains. Compared with the raw material, 56 new volatile flavor compounds were produced after fermentation. Except for aldehydes, the relative contents of other volatile flavor compounds increased, and the contents of alcohols and esters were higher. Compared with natural fermentation, the types and contents of volatiles were also higher after kefir grains fermentation.