Lactobacilli and yeast play important roles in the fermentation of soybean paste.Contents of total acids,pH,alcohol content and quantities of lactobacillus and yeasts were investigated during fermentation of soybean paste with inoculation of Lactobacillus plantarum and Zygosaccharomyces rouxii.Response surface methodology was applied in order to optimize the procedures.The stationary point obtained from response surface model was the saddle point and the ridge analysis was developed to further study the results.Under the optimal condition of temperature 32.4℃,2.87×10~6 CFU/g Lactobacillus plantarum inoculation,4.55×10~5 CFU/g Zygosaccharomyces rouxii inoculation and pH 5.3 at which Zygosaccharomyces rouxii was inoculated,the maximum sensory evaluation score of 91.2 was obtained and the quality of soybean paste was noticeably improved.