Flow entries are forwarding rules generated by controllers and guide switches to process data packets in Software Defined Network （SDN）. Every flow entry is stored in the memory of switches and has timeout， which affects the bandwidth cost in SDN control channel， the memory consumption in switches， and the system’s resource management and performance. As most of the existing SDN performance optimization schemes only have single objective， and do not consider the impact of the types and time of the flow entry timeouts， a multi-objective optimization scheme was proposed based on the dynamic mixed timeouts of flow entries to simultaneously optimize the three objects： the detection of elephant flows， the memory consumption of flow entries in switches， and the control channel bandwidth occupation. In the dynamic mixed timeout， hard-timeout and idle-timeout， two timeout methods of flow entries were combined， and the timeout type and time of flow entries were adjusted in a two-dimensional dynamic way. The NSGA-Ⅱ algorithm was used to solve the proposed optimization problem and to evaluate the impact of different timeout methods and timeout time on the three optimization objectives. The solution set of specific timeouts was combined with the solution set of Bayesian multi-objective optimization algorithm to improve the quality of the solution set. The results show that the proposed scheme can provide a higher detection accuracy， a lower bandwidth occupation， and a smaller switch memory consumption. It significantly improves the overall performance of SDNs.