Abstract: When high energy X ray irradiates materials, the energy will be deposited in the materials, the surface materials will gasify to generate a thermal shock wave inside the meterials; and the finite differential method is usually used for the numerical simulation of the thermal shock wave. In this paper, the method of the Smoothed Particle Hydrodynamics (SPH) method is used to simulate the process of X-ray irradiating materials, As the particles on the surface expand and gasify, the particle size is found to increase tens to hundreds times, and the particles would penetrate. This is caused by the computational formula for the density, which is shown as inappropriate, so this paper abandons the density equation in the SPH method and uses a new formula for calculating the density, and satisfactory results identical with those of the differential method are obtained.