In order to improve the order picking efficiency of B2C electronic commerce enterprise, the storage location assignment in this paper takes into account both the demand correlation and the order frequency. First, we use association rules mining to analyze the demand correlation between items, and form the item combinations. Then we establish a mathematical model for the storage location assignment of the item combinations, the optimization objective of the model is to shorten the total picking distance. And the storage location assignment within the item combination is in accordance with the order frequency. A numerical experiment proves that using this method can get higher picking efficiency.