As a key physical parameter in annular flow, liquid film thickness is crucial to study the behavior characteristics about gas-liquid interface under annular flow conditions. In this study, the narrow rectangular channel is taken as the research object, and air-water were used as the media to conduct annular flow experiments under atmospheric pressure. The cross-sectional area of the narrow rectangular channel is 70mm × 2mm. The PCB liquid film sensor can realize multi-point measurement of liquid film thickness. A total of 10 × 16 measuring points are arranged in rows and columns on the surface of the channel, with a spatial resolution of 4.4mm × 4.4mm and a measurement speed of 1000 frames per second. The results show the fluctuation of liquid film is dominated by the ripple wave at low superficial liquid velocity. The frequency distribution of film thickness becomes sharper because of the increase of gas flow, i.e. the interfacial surface becomes smoother. The liquid film will become thinner with the increase of gas flow, but the effect is reduced when the gas flow reaches a certain value. The liquid film will thicken and the number of disturbance waves will increase as the increase of the liquid flow.