As a novel method to assemble and ignite thermonuclear fuel, shock ignition offers several significant advantages, including low ignition threshold, high gain, and good hydrodynamic stability. In this paper, the mechanism of shock ignition is analyzed theoretically and simulated numerically. First, using one-dimensional planer model, the principles of shock collision in shock ignition are analyzed. Then simulations are performed to study the mechanism of the shock ignition scheme. The simulation results are compared with the theoretically analysis, and the robustness of the ignition scheme in terms of shock launching time is studied.