The paper mainly introduces the characters of active-disturbance rejection controller (ADRC). The ADRC doesn't rely on the accurate math mode of controlled object and it can compensate the system's inner and outer disturbances. According to the characters of ADRC, the paper provides a new method in UAV's longitudinal channel control. The closed loop system can make the pitching angle accurately tracked and controlled without changing its controller's parameters in different flight states. The ADRC also realizes the velocity loop and the pitching angle loop's decoupling control. The result of the simulation shows that the system is good in dynamic response and robustness.