In order to solve the problem of fatal damage of special-shaped and large length-diameter ratio solid propellant charge caused by the inactivation of coating rubber and the softening and deformation of solid propellant due to the high shrinkage temperature of coating material, a water bath heating method with side cavity was designed. According to the principle of natural convection caused by uneven temperature field, the temperature difference is formed, the high-temperature contact time is controlled, and the quality of the coated charge is ensured to be consistent. The dimensional analysis and process simulation analysis of the fluid convection heating in the side heating cavity are carried out, and the results show that the layout of the heater in the water tank heated by the water bath of the coated prototype is reasonable.