In order to study the stress of the flexspline of the cup-shaped flexible harmonic gear transmission system, the spring approximation method and the element contact method were used to carry out 3D modeling of the system. The flexspline deformation and stress distribution at the root of the flexspline were obtained by numerical calculation and analysis, and the numerical calculation results were compared with the experimental results. The results show that the numerical calculation results are in good agreement with the experimental results in both qualitative and quantitative aspects, and it is feasible to use this method to analyze the stress distribution of the flexspline, which can provide reference for the optimal design of the flexspline.