Abstract: Quasi-steady time characterized the response of the water film to the steady state in the passive containment cooling system. It is a key factor in the heat transfer and design of the nuclear safety system. There are two processes during the falling film covering the containment surface. The first process is water flows down, resulting in a wet area; the second is water laterally wet the dry area due to its semi-stability, and the contact angle becomes smaller and slowly increases the coverage area. Coupling of the two processes makes it difficult to determine the quasi-steady time. On the basis of the results of collection tank level, flow and pictures of water film coverage in CAP1400 water distribution experiment, present study proposed two methods to determine the stability of the water film, which are flow balance method and coverage stable method. It solved the calculation problem of quasi-steady time and provided a new way for determining the stability of the water film, and furthermore, relations of relative quasi-steady time with Reynolds number based on different distribution water structures are obtained.