To investigate the problem about effect of formulations and extrusion process of nitroguanidine gun propellant on the extrusion molding size, the mathematical model and boundary conditions of extrusion process for nitroguanidine gun propellant with different formulations were established. The distribution siteration of shear rate field, pressure field and velocity field in different flow process under the condition of different formulations were simulated, calculated and obtained by the finite element method. The results show tha the actual outer diameter of 8.03 mm and simulation size of 8.38 mm of the propellant is basically the same, its error is 4.36%.The error of outer web size (7.89%), inner web size (3.10%), pore size (9.26%) and center pore size (5.36%) etc parameters is within 10%.