This study used response surface methodology to optimize medium composition made by soybean milk remained from Fupi by-product and fermented by Aspergillus oryzae.Based on the study of single factor tests,we identified KH2PO4,sucrose and wheat bran as the important influencing factors through initial screening method of Plackett-Burman.Then the path of steepest ascent was undertaken to approach the optimal region.Box-Behnken design and response surface analysis were adopted to ascertain the optimum conditions as follows:KH2PO4 1.18%,sucrose 0.24%,wheat bran 1.81%,MgSO4 0.2%,NaCl 0.2%,CaCO3 0.02%,tween 0.05% and initial pH 7.Under these conditions,the radio of peptide after fermentation reached 63.31%,which was four times higher than before.