The thickness of liquid film is an important parameter to study the flow characteristics of liquid film. In order to obtain the thickness change rule of liquid film flow, the gravity driven water film flow on the vertical plate surface was measured experimentally.. A falling film flow test rig was designed and built independently. The laminar flow was analyzed by capacitive liquid film thickness gauge. The thickness of the falling film at the distance of 25 cm from the inlet changed with time. The experimental results show that in the time domain, the liquid film thickness of the observation points is constantly changing. When Re=297, the flow liquid film in 0 to 70 s fluctuates smoothly, and the average thickness is close to 168.5um. When Re = 462 and Re = 627, the fluctuation is strong in 0-40 s, and the liquid film flow tends to be stable in 40-70 s. At Re=814, the liquid film fluctuated more sharply and the fluctuation amplitude increased when 0-70 s was measured. The thickness of liquid film has a positive linear correlation with Reynolds number. When Re = 814, the liquid film thickness at each time point is above 360 um, which is higher than that at the other three different Reynolds numbers. When the Reynolds number is high, the fluctuation of the liquid film is more intense. The reduction of Reynolds number is conducive to the formation of a smooth liquid film. The flow liquid membrane shows instability.