In order to solve the problem of the traditional sour liquid producing methods in the extraction ratio and quality,the experiment uses the fermented liquid of lactobacillus paracasei which is separated from the sour liquid of Sweet Potato,to replace the traditional sour liquid to separate mung bean starch.Through orthogonal test,we find out that the major factors that influence the effect of Lactobacillus paracasei are pH,temperature,proportion of solid and liquid in the defibrination and the volume of the fermented liquid.The sequence of the primary and secondary factors is the proportion of solid and liquid during the defibrination,temperature,the volume of the fermented liquid and pH.And the optimum parameters are that the volume of the fermented liquid is 33 %,pH is 7.5,the temperature is 60℃ and the proportion of solid and liquid in the defibrination is 1∶ 3.