A solidification process of casting explosives in shell is studied in this paper. Heat enthalpy model and Darcy law are used to simulate the process of phase change and mushy zone. SIMPLER algorithm is used to solve the mass, momentum, and energy equations to obtain the temperature and velocity distribution of the field. Both the conduction and free convection effects on solidification are analyzed. Numerical results show that the solidification time is increased and the temperature distribution is more homogeneous if both the conduction and free convection effects are taken into account during solidification.