By using Plackett-Burman and response surface methodology,we studied the medium optimization of purification of stachyose by fermentation.A Plackett-Burman design was first used to evaluate the influence of six related factors,and yeast extract,casein peptone,NaNO3 were affirmed the important factors in the medium,and then the path of steepest ascent and the Box-Behnken design were used for further optimization on these three factors.By solving the quardratic regression model equation,the optimal concentrations of the variables were determined as: yeast extract1.38%,casein peptone 0.82%,NaNO30.48%.Under the optimal culture condition,the purity of the stachyose increased from 85% to 91%.