Software-Defined Network (SDN) is developing rapidly for its benefit of programmability. However, new challenges also appear. One of them is while we are applying distributed controllers in SDN networks, we must consider the consistency problem. In SDN networks, especially in a multi-controller architecture, it is a great challenge to maintain a global view consistency of the networks among all controllers, which is also key to issue flow regulations. Lacking of consistency in packets, flows, and networks level may result in serious errors. Besides, consistency problem also exists in data plane. How to keep all switches executing a same set of rules to avoid errors is often discussed. In this paper we’ll conclude the different situations of consistency problems and provide the related research solutions. The methods in the paper not only regard to consistency of control plane, but also data plane. At last, we also introduced the two methods to evaluate the performance of consistency, those are strong consistency and final consistency.