An active disturbance rejection controller(ADRC) is applied to quadrotor aircraft attitude control. Firstly, the extended state observer(ESO) is analysed and the improved faln function is got based on improvement of the ESO's nonlinear fal function. The convergence of the improved ESO is demonstrated. Then, compared with traditional fal function, the improved faln function can obtain better effect of “big error, small gain”. Finally, the improved faln function is applied to quadrotor aircraft attitude, and simulation results show that the proposed function is superior to the traditional fal function on disturbance rejection.