In order to explore the factors affecting the size fluctuation for the variable-burning rate propellant in the co-extension process,numerical simulation method was used to simulate the flow under different operating conditions. Results show that the screw velocity fluctuation has more influence than the entrance pressure fluctuation on the size fluctuation of the variable-burning rate propellant,especially have an influence on the interface radius. The entrance pressure fluctuations have an impact on the size fluctuation of the variable-burning rate propellant,especially when the screw velocity combination is 20：30. The decrease of the entrance pressure fluctuation of the inside layer and outside layer materials, and increase of the entrance pressure of the inside layer materials properly, and reducing the solvent content of the inside layer materials and enhance the solvent content of the outside layer materials will be helpful to improve the precision of grain size for the variable-burning rate propellant.