A new aquatic-aerial amphibious unmanned underwater vehicle with 4 tilt-rotors is introduced, depending on the application background of the vehicle, the departure process of the vehicle from water to air (water surface departure) is an important operational task. The departure process in the water-surface condition is influenced by propeller power attenuation, various buoyancy, and fluid dynamics between different water and air, presenting great challenges for vehicle surface departure motion control. In this paper, dynamic model of vehicle near the water surface is proposed. Based on the dynamic model, the departure process on water surface is designed and the proposed process effectively improve the thrust-weight ratio near the surface. The stability and load capacity of the vehicle during departure are enhanced. Finally, the successful experiments show that the departure process control is effective.