A mathematical model of film thickness uniformity of planar three-stage planetary fixture is established. Based on this model, a computer simulation program is compiled to study the film thickness uniformity of planar three-stage planetary fixture used in 1.8 m large diameter coating machine. Taking Opt-run OTFC-1800-DCI coating machine as the simulation object, the effects of evaporation source characteristics, planetary fixture inclination and planetary orbit radius on film thickness uniformity are analyzed. The simulation results show that the inclination of planetary fixture is the main factor affecting the film thickness uniformity of planar three-stage planetary fixture α= 64°, the film thickness distribution is the most uniform, and the non-uniformity is 0.1%. The radius of planet orbit also has an effect on the uniformity of film thickness. When the radius of planet orbit is 665 mm, the film thickness distribution is the most uniform, and the film thickness uniformity is controlled within 1%.