With the consideration of Orowan strengthening effect, a new constitutive model of particulate-reinforced magnesium matrix nanocomposites for thixotropic plastic forming was derived by analyzing relationships among parameters (stress , strain , strain rate , liquid phase rate fL, volume fraction of nano-reinforcement fp, temperature T). The multiple nonlinear regression method was used to determine the parameters in the constitutive model. The findings indicate that there exists a specific constitutive relationship of thixotropic plastic forming of nanocomposites and the corresponding model was proposed. The calculated values obtained from the constitutive model had good agreement with the experimental results. As a result, the constitutive model provides references for the thixotropic forming theory, simulation and fabrication of nanocomposites.