Dynamic soaring is a special flying technique designed to allow air vehicles to extract energy from atmosphere. Based on the established dynamic model of the aircraft, the energy transformation of unmanned aerial vehicles (UAVs) in a gliding cycle of the known gradient wind field was analyzed. The method of subsection analysis was introduced and a dynamic soaring cycle was divided into four phases: climbing with headwind, turning in high altitude, gliding with tailwind, turning in low altitude. Turning in high altitude was the most critical phase in the dynamic soaring cycle. By using the calculation method which combines the three-dimensional space path with the two-dimensional plane projection, the motion equation and energy transformation equation of UAV in the phase of turning in high altitude were achieved. Based on the equations, the parameters which influenced the energy extract of UAVs from gradient wind field and which resulted in the energy loss because of the wind drag were analyzed. The conclusion provides theoretically guide for the UAV’s maximum extracted energy from gradient wind field and the decreasing of its energy loss. Besides, the dynamic soaring simulation was designed and results show that the phase of climbing with headwind and the early stage of turning in high altitude are the mainly periods to absorb energy from wind field. This conclusion is meaningful to direct the dynamic soaring without thrust.