Software-defined network (SDN) has received great concern because of its prominent advantages in some aspects such as high flexibility, programmability and dynamic management, etc. Recently, some works begin to apply the concept of SDN in mobile networks to make the networks more open and agile. However, exploiting SDN in wireless networks is still in the initial stages. The potential challenges of applying SDN in an operation wireless network remain unexplored. In this paper, we first summarize related researches and explain the advantages brought by adopting SDN in wireless networks. Then, potential challenges are analyzed base on operation data in a large-scale WLAN. What can be found is that the challenges are either different or more severe than problems of SDN in wired networks because of special characteristics in wireless and mobile environment. In addition, several existing proposals and potential methods concerning on scalability problems are introduced. The analyses in this paper could provide some hints when applying the SDN concept into an operation wireless network.