The process of laser ignition of RDX was calculated using numerical simulation. The results show that the gas phase is ignited with the accumulation of reactant and the increase of temperature and reaction rate in the gas phase after undergoing a series of processes of heating, melting, decomposing, vaporizing and flowing into gas-phase for RDX. The effect of environment pressure and laser power density on ignition delay time and critical ignition distance was discussed. The calculated ignition delay time is consistent with the results reported in previous literatures.