The development of internal stress in injection molded parts is analyzed. Different from other researches, this study uses a new modified Maxwell model to calculate the internal stress. On the basis of the creep experiments of injection molded parts, a non-linear constitutive equation is proposed. Non-linear finite element equation to calculate the internal stress is derived. By means of this model, the internal stress of an injection molded polystyrene plate is simulated. The effects of mold wall temperature, cooling time and packing pressure on the development of internal stress are investigated. The predicting results are in good agreement with experimental data.