Monitoring networks in cloud is important and complex. In the cloud computing, network security equipment does not only monitor external network traffic, but also review the traffic within the network to resist attacks. There are some researches that introduce SDN and OpenFlow to route the traffic to security device. However, these methods are limited by the number of devices of the same type, and their performance is hence affected. In this paper, we propose a new flexible cloud computing monitoring framework based on OpenFlow. And we also propose the corresponding algorithm to improve the performance and efficiency. By doing so, we can monitor the network traffic efficiently and flexibly.