To solve the order picking problem of automated storage and retrieval system (AS/RS), an optimization model was established based on least time according to AS/RS machinepsilas horizontal and vertical motion. The disadvantages of genetic algorithm (GA) and immune particle swarm optimization (PSO) in solving the above model were presented. To get better optimization result, an algorithm which combined GA and immune PSO is presented. The calculated results show that the algorithm is operable and versatile. Simulation and practice show that this new mathematic model based on least time is more effective and applicable than the result of the model based on shortest distance by the same optimization algorithm.