Based on Reynolds-averaged Navier-Stokes equations and k-ω turbulence model, the numerical simulation of the ignition transient of a single chamber double thrust solid rocket motor with large aspect ratio is carried out. The simulation results show that gas flow congestion occurs in the early stage of ignition, and a fluid throat is induced in the narrow port upstream of the transition region, which causes intense pressure oscillation in the transition region and produces a high pressure boost rate. At the same time, the flow separation between the downstream gas and the wall occurs, resulting in a secondary ignition point, which speeds up the flame propagation process.