Abstract: Based on the viscoelastic theory of solidified polymer，a new four-element mechanical model in series was proposed and its non-linear constitutive equation was derived to calculate internal stress of injection molded articles.The equation illustrates,the instantaneous viscoelastic behavior of polymer and its effect on internal stress state of the product in the cooling processThe equations for calculating elastic modulus and viscosity of polymers were givenThe internal stress of an injection molded polystyrene plate was simulated by means of the new modelThe results are in good agreement with published researching conclusions about the structure and mechanical properties of polymers, which show that the model is accurate and feasible.