In order to balance the number of outbound and inbound tasks of each aisle and improve the working efficiency of the multi-tier shuttle system, three principles of storage location assignment were put forward: the principle of minimum correlation degree, the principle of equalization of product items and the principle of equalization of tasks, which were expected to be followed when storing products. The association rule analysis method in data mining was used to analyze the historical order data, and the correlation degree between different product items was obtained. According to the above three principles, simulation experiment of storage location assignment was carried out and the variance of the number of tasks in each aisle was calculated. The simulation results showed that the variance of the number of tasks of each aisle is the smallest when the three principles were adopted synthetically. It could be concluded that the three principles we put forward were of use to balance the number of tasks of each aisle in multi-tier shuttle system.