In order to improve the efficiency of order picking in robotic mobile fulfillment system, a slotting optimization method which takes into account the the correlation of goods and the total frequency out of warehouse of goods on the shelf is proposed. The problem of slotting optimization is divided into two stages: Deciding which goods to put on which shelf and which shelf to put in which position in the warehouse. In order to avoid congestion, a decentralized storage strategy is adopted. Two mathematical models are established to minimize the total number of shelves movings and the total picking distance of robots, and a two-stage heuristic algorithm is designed to solve the models. Finally, the simulation experiment is carried out based on the historical order data of an online pharmacy. The experimental results show that the slotting optimization method proposed effectively improves the efficiency of order picking.