Abstract: Streptococcus thermophiles was used as a modified strain to improve natto by secondary fermentation.The sterilized soybeans were inoculated with 2% natto inoculum and fermented 24 h at 37℃.Through single factor and response surface experiments,based on the fermentation temperature,fermentation time and inoculum of Streptococcus thermophilus,with sensory score and nattokinase activity as indicators,the most effective conditions for secondary fermentation of Streptococcus thermophilus was studied.The results showed that the best improvement was obtained when the fermentation temperature was 42℃,the fermentation time was 18.6 h,and the inoculum volume was 1.5%. The sensory score was 8.0 score and the nattokinase activity was 1964 U/g. The improvement effect was good,the bad odor of natto decreased,and the activity of nattokinase increased.