Abstract: The film thicknesses of liquid swirl injector under different backpressures were studied theoretically and numerically.A loss coefficient model including friction effect on liquid/gas interface was established based on classical theory.It shows that the theoretical model is not sufficient to show variations of film thickness under different backpressures.The numerical simulations were conducted with a swirl axi-symmetric model to solve Navier-Stokes equations coupled VOF(volume of fluid) equation on an adapted grid,showing that the decreased mean velocity in the transition region near liquid/gas interfaces in liquid film is a contributing factor to the increase of the film thickness.