The medical business of China Post Logistics is developing day by day. China Post Logistics have been using the Parallel partition picking mode for a long time, but too many shortages have appearing in these two years, especially in the order picking cost and efficiency. This paper analyzes the current order picking mode of China Post Logistics and compare with the Parallel partition picking mode. By analysis the advantages and disadvantages of these two modes and combine with the actual situation, we choose the serial partition relay picking model as the picking mode of CPL in medical logistics area. And then it optimizes the order picking route in view of the current deficiency combined with the use of Ant Colony Optimization (ACO). The example simulation result shows that this optimizing is effective and the order picking cost decrease 17.36% and the route decrease 9.80% than that as before. This research not only to Chain Post Logistics but also to other logistics company which runs the medical business has certain reference.