Coarse-grained (CG) models based on molecular dynamic were developed to analyze the flowing process of polymer melt in nano-sized channels on a mesoscale. Firstly, atomistic models of amorphous isotactic Polymethyl Methacrylate(PMMA) were built by Materials Studio， and initial CG models of different mapping centers were then developed based on the models mentioned above. After the structure optimization and kinetic analysis of the atomistic models, the statistic law of potential energy of the system was obtained and the initial force field forms were calculated by Gaussian fitting function and Boltzmann inversion method. Then, the formula of force field was iterated and modified according to the relative strength and the relation of bond, angle and nonbonded potentials and an optimized CG model was obtained successively. Finally, the static properties of the CG model, such as mean square end-to-end distance and mean square radius of gyration, were compared to those of atomistic simulation, and obtained relative deviations are 0.68% and 6.6%, respectively. The results demonstrate that the mapping center has impact on the models and the optimized CG model reproduces the atomistic model well. It can be used to analyze and explain the flow and mass transfer behaviors in the nano-injection molding process.