As a mission-critical network component in software defined networking (SDN), SDN control plane is suffering from the vulnerabilities exploited to launch malicious attacks, such as malicious applications attack, modifying flow rule attack, and so on. In this paper, we design and implement mimic network operating system (MNOS), an active defense architecture based on mimic security defense to deal with it. In addition to the SDN data plane and control plane, a mimic plane is introduced between them to manage and dynamically schedule heterogeneous SDN controllers. First, MNOS dynamically selects m controllers to be active to provide network service in parallel according to a certain scheduling strategy, and then judges whether controllers are in benign conditions via comparing the m responses from the controllers, and decides a most trusted response to send to switches so that the minority of malicious controllers will be tolerated. Theoretical analysis and experimental results demonstrate that MNOS can reduce the successful attack probability and significantly improve network security, and these benefits come at only modest cost: the latency is only about 9.47% lower. And simulation results prove that the scheduling strategy and decision fusion method proposed can increase system diversity and the accuracy of decisions respectively, which will enhance the security performance further.