A model of thermal stress in double-layer optical dielectric films on circular substrates was established based on the theory of double-layer composite beams. Here, considering the boundary conditions including force balance and bending moment balance, the distribution of stress and strain in the double-layer film-substrate system was analyzed following equivalence manipulation to determine a detailed formula for calculating the thermal stress in the equivalent film and substrate. The derived formula was not only effective in analyzing the stress and strain of the double-layer film-substrate system but was also applicable for predicting the distribution of thermal stress in the periodic elastic multilayer film-substrate system. According to the actual radius of curvature of the substrate measured via a profilometer before and after the deposition of the HfO2/SiO2 double-layer films, the obtained residual stress of the film was - 79.33 MPa, whereas the thermal stress of the film was calculated to be -52.59 MPa using the theoretical formula. The calculations of the theoretical model were similar to the experimental results when the smaller intrinsic stresses were neglected and the double-layer film was only of nanometer thickness, thus verifying the effectiveness of the double-layer film-substrate model.