Abstract: For the problem that a quadruped robot loses its balance when walking into a trench in the fields or on a downward non-structured step,a balance control solution,aiming at counteracting the vertical inertial force is proposed,based on the biological neural reflex mechanism.The detected abrupt change of the body pitch angle will trigger the postural reflex, which will produce coordinated movements of the four legs to adjust the robot's attitude rapidly to counteract instantaneous imbalance caused by inertia.Physical experiments are designed comparatively using a 8-DOF quadruped robot.The robot successfully crosses over a 1.2-leg-stride wide trench and walks down a 0.4-leg-length high step.