Abstract: Falling film is widely occurred in the safety system for the Advanced PWR Nuclear Power Plant（e.g.AP1000） and seawater desalination using nuclear energy.In this paper,the instability of falling film was experimentally studied by Shadowgraph technique.The disturbance wave pattern in a wide range of incline angle（β=0°～90°） were obtained in shadowgraph format.It is indicated that gravity is important for the development of disturbance waves.And moreover,the numerical image processing was applied to analyze the characteristics of disturbance waves on falling film,the development of disturbance waves was analyzed quantitatively and the importance of gravity for disturbance waves was proved again.The dependence of disturbance wave evolution on Reynolds number and flow distance in the main stream direction was obtained and meanwhile four distinctive regions were unveiled.It is indicated that Re has some influence on the regions distributing of disturbance waves.At last,the frequencies of disturbance waves were obtained by spectrum analysis,which accords with the experimental data.