The effects of temperature, pH, substrate concentration, enzyme concentration on soy protein hydrolyzed by papain were researched with the degree of hydrolysis as indicator through single-factor and orthogonal experiment. The condition was optimized. The result showed that the order of the factors effecting the hydrolyzation was temperature, pH, enzyme concentration and substrate concentration. The optimized parameters were temperature 60 ℃, pH 7.5, enzyme concentration 3.5%, substrate concentration 5%. Hydrolysis for 40 min under the optimized condition，the degree of hydrolysis of soy protein by papain could reach to 6.58%, the average number of amino acids of soybean peptide was 15, and the average molecular weight was 1 875.