In order to solve the problems of oscillation, fluctuation and controller parameter tuning in power systems, this paper presents a parameter tuning method of active disturbance rejection control based on Nyquist curve. Firstly, the active disturbance rejection controller is equivalent to a two degree of freedom control structure, and then Nyquist curve is drawn. The corresponding controller parameters can be calculated according to the expected left asymptote or intersection point, and the corresponding parameters can be adjusted to obtain the expected response speed. This method is intuitive and concise, convenient for industrial field operators to tune parameters, and can be extended to any order of objects and controllers. The simulation experiment on a denitrification system shows that compared with the PID parameter tuning method for second-order oscillation models in relevant literature, the adjusting time, the tracking IAE index and the disturbance rejection IAE index of the proposed method is 34.58%, 62.60% and 2.24% of the PID scheme respectively, which has superior control performance and anti-interference performance.