Wave structure of film of liquid in a circular gas-liquid stream was studied with help of Laser Inducted Fluorescence (LIF) method in a channel with inner diameter 15 mm. With aim to explore the process of formation of perturbation waves experiments at small distance from the entrance to the channel were conducted, the length of the working sector was 100 mm. It is ascertained that at high speed of gas and consumption of liquid perturbation waves are present in the measurement region, and also become dominant wave structures on the surface of film of liquid. It is shown that formation of perturbation waves happens thanks to confluence of small-scale high frequency waves that appear at the entrance. Quantitative exploration of formation of waves was being conducted with help of spectral method. Transfer of energy to the side of low frequencies was found. Significant growth of waves’ speed was found.