The PV-PQ bus type switching is a typical complementarity problem， which reflects the discipline of voltage control by reactive power in power flow computation. By introducing the complementarity theory， the strict mixed complementarity model for PV-P Q bus type switching is constructed， and it is solved successfully by the modern interior method integrated with a complementarity relaxation method. Analysis on PV-PQ type switching logic and comparison with the complementarity model show that the type switching logic may cause the bus type misidentification and obtain static voltage unstable points. As a result， the related numerical and identificate divergence may not correspond to system voltage unstable point. All these issues can be involved in the complementarity model to improve the reliability of the results. The conclusions are verified by simulation of IEEE standard systems and a real system.