Effects were studied, of which Na2SO3 as reducing agent or surfactant including Tween 80 and soybean lecithin, on soy protein hydrolysis process, product yield and foaming capacity by papain hydrolysi. The results showed that adding suitable amount of Na2SO3 and soybean lecithin not only can improve the hydrolysis process significantly, but also can improve the yield of soluble hydrolysate, and the product has high foaming capacity and foam stability. Amino acid analysis showed that the content of hydrophobic amino acid in precipitation was higher that in supernatant. It means that aggregation and precipitation of hydrolysis processes have close relation to hydrophobic interaction.