In this paper, the structure of the active disturbance rejection control technique is described in detail, including the tracking differentiator, extended state observer and nonlinear state error feedback. Typical algorithms of each part are given as well. In order to control the attitude of the quad-rotor robot as we desired, we design two kinds of ADRC, including the continuous ADRC and the discrete ADRC. Simulations were carried out based on Simulink. After parameter adjustment, the simulation results show that the ADRC can meet the need of control accuracy and swiftness of response. It also indicates that the controller has strong robustness and anti-disturbance performance, which can control the nonlinear coupling systems effectively.