Channel interference caused by the Access Points (APs) of a secondary Multiple Input Multiple Output (MIMO) WLAN, is considered in this paper when they cognitively access to the TV White Space (TVWS) of primary network. This paper constructs a typical scenario where heterogeneously exists TV broadcast primary network and secondary network based on WLAN, and analyses the interference of Cognitive APs (CAPs) on primary network and the channel interference among the APs in secondary network. In order to reduce the channel interference, and meet both the communication Quality of Service (QoS) in primary network and maximum throughput of secondary network, the CAP channel assignment problem is modeled as a mixed integer linear programming problem. A joint optimal power control and channel assignment algorithm called JOPCCA is proposed. The optimal solution is obtained by Karush- Kuhn-Tucker (KKT) theory. A reasonable spectral sharing scheme is designed for CAPs access to the mixed Industrial Scientific Medical (ISM) and TVWS bands. Numerical simulation results show that the proposed algorithm can effectively reduce the channel interference of CAP on primary network and the channel interference among APs in secondary network, allocate the mixed ISM and TVWS channels reasonably and optimize the transmit power of CAPs.