The dense deployment of wireless access points (APs) makes AP association an important problem. Currently, AP association is solely based on the signal levels of APs. However, this approach fails to consider the heterogeneous nature of APs, and the variety of user demands (bandwidth, security, delay, etc). In addition, distributed AP association cannot achieve network-level load balance. To address the issue, this paper proposes a centralized AP association model based on the software defined network (SDN). This model considers the objective of network administrator and wireless clients simultaneously, and can optimize the load balance of APs as well as satisfactory of clients. Ant colony algorithm is used to solve the model, and simulation is performed to validate the algorithm. Results show that user satisfactory factor as defined in our model increases from 54.5% to 86.8% under heavy load, and to 94.1% under light load. In addition, the load balance of APs also improves remarkably.