Theoretical analysis and experimental study on the thickness distribution of Ta₂O₅ film evaporated on the inner-face of a hemispherical substrate are demonstrated. It is derived that the value of n/R and L/R influence the film thickness distribution (where R is the radius of the hemisphere, n and L are the horizontal distance and vertical height between the evaporation source and the center of the hemisphere, respectively). The whole hemispherical substrate can be coated when n≤L+R, otherwise there is a “blind area” on the substrate when the substrate is self-rotating. A hemispherical composite substrate with a radius of 200 mm is coated with Ta₂O₅ protective film under a certain configuration, the thickness of Ta₂O₅ film at the edge is 0.372 times the film at the vertex which shows that the evaporation characteristics of Ta₂O₅ tend to be a point source.