To study the deformation and stress of ring-flexspline in harmonic gear under actual working conditions,an interference fit model is applied to solve the problem of assembling between the wave generator and flexspline with large deformation. Further more experiments are performed to test the validity of this method. Then the finite element numerical calculations of the whole harmonic gear are obtained under no-load and rated load on the base of the model. At last,the deformation and stress distributions law of flexspline in the no-load and load are analyzed by data processing. The results show that the stress distribution of the flexspline has its minor value along the neutral layer of flexspline ring,and the maximum stress is located on the tooth root which mainly is tangential. The deformation and stress increment of flexspline under rated load are0. 46% and 93. 6% respectively relative to the no-load. It also shows that the load has little effect on flexspline deformation,but a strong influence on its stress. The finite element analysis results which fit well with related literature calculations proved that the interference fit model is effective,so it can provide a more precise method to study harmonic gear drive in accordance with the actual working conditions.