Abstract: During nuclear power plant construction, commissioning is a very important stage. Through commissioning it finds the problems of the whole system, verifies the correction of design, equipments, and the installation, so to guarantee plant work as the performance requirements. This article states the commissioning purples, commissioning program management, commissioning results analysis and treatment. Using the NSSS ASG commissioning as example, it states the main content of commissioning and utilizes the flowmaster software handling the problem of deaerator flowrate excessing base requirement. That deals with the problem quickly and fast.