The motion mechanism of an underwater glider is the same as its aeronautical counterparts. It uses small changes in its buoyancy in conjunction with wings to convert vertical motion to horizontal, and thereby propels itself forward with very low power consumption. We focus on the forces acting on the glider during its steady motion, which are shown in Fig.1. Let the net weight of underwater glider in air be W , the lift be L, the drag be D , the gliding angle be ξ , the attack angle be α and the pitch angle be θ , respectively. Then the underwater glider moves forward with the gliding angle