Abstract: A theory study on film interface stability on the wall of cyclone separator in viscous condition was performed in this paper. The dynamics boundary condition and kinematic boundary condition in the rotational flow field were obtained based on the stress analysis of film in viscous condition. Then the momentum equations and continuity equations of gas-liquid fluid were linearized by substitution of potential function. The dispersion relation of film interface wave in viscous condition was established by the combination of boundary conditions and the linearized equations. The judgment criteria for film interface stability was obtained. A code was developed according to the model of film motion and dispersion relation. The effect of liquid viscosity on the film interface stability in the cyclone separator was obtained based on the analysis of film interface stability in viscous condition.