In cloud data centers, massive concurrent access to servers is very common. Traditional network architecture is difficult to control traffic forwarding globally, and expensive load balancers need to be configured to deal with this application scenario. The SDN（software defined network）can globally control the network status through the controller, and use the switch as a load balancer, thereby reducing deployment costs. A server load balancing strategy based on OpenFlow switches is proposed in this paper. Service requests are partitioned and mapped through multi-address directed flow table, and the number of active connections is used as a load evaluation parameter, find the optimal load redirection scheme through ant colony algorithm. During load migration, single-address directed flow table is used to ensure orderly forwarding of traffic at different stages. This strategy can effectively control the size of flow table and has better performance than traditional equalization strategies.