We explored the failure mechanism of rock under ultra-high strain rates using 3D numerical modeling of the light gas gun test. Based on numerical results, it concluded that mesoscopic hydro-compressive failure rather than mesoscopic shear or tensile failure is the main mechanism of rock failure under the condition of shock compressive loading. The shock wave, indicated by the stress signals of two stress gauges in the rock specimen, can be well reproduced by numerical simulation with the quasi-static rather than the dynamic elastic parameters. The simulation results indicate that the compressive shock wave involves a compressive failure loading process similar to that shown in the conventional uniaxial compressive failure test rather than the ultrasonic test. A mesoscopic-rate-dependent failure model was developed to take the dynamic effect into account. Our results revealed that larger rock porosity could result in an decrease in dynamic strength and dynamic effect under shock compressive loading.