As the popularity of mobile applications, Wi-Fi has become one of the major access methods for many users. When a large amount of users move in public places together, such as classrooms or meeting rooms, severe load imbalance of Wi-Fi Access Points (APs) and unfair bandwidth allocation are likely to occur consequently. Software Defined Networking (SDN) is a new networking paradigm which allows the network administrators to write programs for controlling the behaviors of network devices. In this paper, we propose an adaptive load balancing scheme through association control in wireless software defined network. The proposed scheme consists of an event detection mechanism and an adaptive load balancing algorithm on controller. In our algorithm, controller can derive an optimal association solution based on the traffic load and number of users on each AP. To observe the effects of population distribution and user mobility, we propose a simulation model to investigate the performance in terms of average AP load, user bandwidth and user throughput. Our simulation results show that our scheme has better performance than other three methods and performs better in imbalanced environment.