Underwater vehicle plays an important role in ocean engineering. Depth control by fin is one of the difficulties for underwater vehicle in motion control. Depth control is indirect due to the freedom coupling between trim and axial motion. It is included the method of dynamic analysis and lift-resistance-coefficient experiment and theory algorithm. Considering the current speed and depth deviation, comprehensive interpretation is used in object-planning instruction. Expected depth is transformed into expected trim. Dynamic output fluctuation can be avoided, which is caused by linear mapping of deviation. It is steady and accurate for the motion of controlled underwater vehicle. The feasibility and efficiency are testified in the pool and natural area for experiments.