Abstract: To realize stable walking of the biped robot on flat ground, a nonlinear impulse thrust is applied at the heel of the support leg, while driving torques are applied at the hip and ankle joints, a walking dynamics model is established to analyze the periodic gait of the robot. The nonlinear differential equations in the continuous phase are linearised using a Taylor series, and the existence and stability conditions of the period-1 gait are obtained using the Poincaré map. The Flip bifurcation phenomenon of the periodic gait is further analyzed using the bifurcation theory. Simulation results show that the biped robot can walk stably on flat ground by adjusting the impulse thrust.