Software-defined networking (SDN) is an emerging paradigm that decouples control and data plane. The control plane advocates a centralized approach of network control, which provides both flexibility and high efficiency, but suffers from serious scalability problems. The state-of-art works on the scalability of control plane in SDN is surveyed in this paper. First, the factors that influence the scalability of control plane are analyzed, and some approaches for improvement are revealed accordingly. Next, current researches and solutions are introduced from the following 4 aspects:Cache optimization on data plane, high-performance controller, distributed control plane and control resource allocation optimization. Finally, a brief summary of current technology is provided along with a discussion on future research directions.