The orientation of postharvest cabbage is the key to the automatic operation of the postharvest processing system, which provides the position and posture conditions for the subsequent root trimming, detection and grading, etc. Aiming at the problems of low efficiency and high labor intensity of artificial continuous orientation of postharvest cabbage, an automatic orientation method based on the moment of inertia for postharvest cabbage was proposed, and the “Zhonggan 15” cabbage was taken as the research object. Firstly, the simplified geometric model of postharvest cabbage was established, and the moment of inertia in the three principal axes of inertia and the motion stability of postharvest cabbage during orientation were analyzed. Secondly, the orientation simulation model was established, and the variations both of displacement difference and included angle with time of 26 initial attitudes of postharvest cabbage during orientation were simulated and analyzed based on ADAMS software, to verify the feasibility of the method. Finally, automatic orientation test platform was built, and single factor test and orthogonal test were carried out with the roller diameter, axial clearance, radial clearance and angular velocity as test factors, and the included angle between the central axis of postharvest cabbage and the roller axis and the orientation success rate as test indexes. It can be seen from the theoretical analysis and simulation results, postharvest cabbage gradually tended to rotate around the central axis with the smallest and only moment of inertia during the orientation, and the movement state was stable. That was to say, the automatic orientation of postharvest cabbage by this method can be realized. The experimental results showed that the primary and secondary influence order of test factors on the included angle between the central axis of postharvest cabbage and the roller axis was roller diameter, angular velocity, axial clearance and radial clearance. And the primary and secondary influence order of test factors on the orientation success rate was angular velocity, roller diameter, axial clearance and radial clearance. The optimal parameters of roller diameter, axial clearance, radial clearance and angular velocity were 80mm, 80mm, 70mm and 6rad/s, respectively. On the condition of the optimal parameter combination, the validation test was carried out. The average included angle between the central axis of postharvest cabbage and the roller axis was (6.72±1.23)°, the time needed for orientation was 6.8s, and the orientation success rate was 96%, which met the requirements of automatic orientation and subsequent high-throughput processing for postharvest cabbage, and also provided theoretical basis and technical support for orientation operation of postharvest processing that can be applied in other irregular spherical fruits and vegetables.