Abstract: In order to explore the characteristics of the deep-sea AUV (autonomous underwater vehicle) unpowered diving motion and improve the prediction accuracy of diving motion, the interferences of ocean current and buoyancy changes are considered, and a simulation method of AUV steady unpowered diving motion is established based on its space motion equation. By changing the control input parameters, the variation laws of state variables of AUV with respect to the net negative buoyancy, the position of the center of gravity, the rudder angle and the marine environment are revealed. The results show that because of the ocean current and buoyancy changes, the motion state of AUV will change to accommodate the new flow, but the motion posture is always stable. The unpowered spiral diving can greatly improve the drift distance of the AUV in the horizontal plane. Under the same environment interference, the faster the AUV dives, the shorter time it takes, and the smaller the drift distance is.