To improve the Natto flavor, in this experiment, different ratios between black soybean and soybean as raw materials were used to explore the suitable fermentation conditions for Natto producing. The results of orthogonal test show that the best condition is: the ratio of black soybean to soybean is 1:4, boiling for 30 min, Bacillus natto inoculation amount 10%, fermentation temperature 38°C and fermentation time 13 h.