Since the development of ADRC(Active Disturbance Rejection Control), it has achieved fruitful results in academic research and application fields. However, ADRC has limited its application range due to its multi-parameter adjustment problem and the difficulty of parameters tuning. So it is necessary to study the tuning of its parameters. Online setting method has the advantages of fast setting speed and high precision, but most of the current research is offline setting. This paper analyzes the second-order ADRC system and simplifies its parameters, establishes a variable domain fuzzy controller to adjust the parameters online, and simulates the ordinary second-order nonlinear system as an example. The simulation results show that the variable domain fuzzy control method can realize the online parameter adjustment, has the characteristics of online adaptive adjustment. The control performance of the control system after online adjustment is improved.