Two-dimensional axisymmetric interior ballistics simulations in the projectile launch system utilizing granular or long slotted tubular solid propellant are carried out using solid/gas two-phase fluid dynamics code of Eulerian-Lagrangian approaches. The simulation results are compared with experimental data for tubular solid propellant, for validation. The conditions ofpropellant grain size and shape affect the energy release rate of solid propellant and then the projectile kinetic energy at the muzzle. An appropriate burning surface area condition of propellant grain exists, where the projectile can obtain the maximum kinetic energy from the released energy of solid propellant. From the simulation results the guideline of the grain size design is proposed.