Abstract: To investigate the three-dimensional effects and asymmetric flame propagation during the ignition transient of solid rocket motors, a numerical method was established by coupling three-dimensional unsteady chamber flow, gas-solid heat transfer, transient heat conduction inside the propellant, and an Arrhenius-law-based surface combustion response model . Under the same operating conditions, comparative simulations were carried out using a symmetric model and a full model, and the predicted head-end pressure histories were validated against experimental data. The results show that both models can capture the overall pressurization trend during ignition, whereas the full model agrees better with the experiment results. Due to the lack of realistic radial three-dimensional flow motion, the symmetric model produces a faster axial propagation of the high-temperature jet in the front cylindrical section, leading to earlier flame development and pressure rise. In contrast, in the rear post-step section, flow in the full model features stronger radial diffusion and circumferential gas filling, resulting in faster high-temperature propagation and a more reasonable pressure growth process. The evolutions of propellant surface temperature and ignited regions further indicate that solid propellant ignition involves an evident thermal lag, and that flame propagation has significant three-dimensional asymmetry. The present results can provide a reference for ignition analysis and model selection of solid rocket motors.