Abstract: For the parameter tuning of linear active disturbance rejection controller (LADRC), the bandwidth method has some problems, such as the difficulty of control gain selection and the time-consuming of observer bandwidth trial, etc, a method to obtain LADRC parameters directly by designing PI controller and first-order low-pass filter was proposed. Based on the analysis of two-degree of freedom(2-DOF) equivalent structure of active disturbance rejection control (ADRC), the corresponding relationship between the parameters of in loop controller and PI controller was derived, and the parameters of LADRC controller were obtained by solving the specific cubic equation of one variable. Taking buck converter as an example, the dynamic performances of PI and LADRC under disturbance were analyzed, and an experimental platform was built to verify the effectiveness of theoretical analysis. The results show that compared with PID control, LADRC control can reduce the adjustment time of start-up process by 240 μs, and the overshoot decreased by 56%. During the loading process, the adjustment time is reduced by 100 μs, and the overshoot decreased by 1.2%. In the process of load shedding, the regulating time is reduced by 50 μs. The proposed tuning method can effectively improve the dynamic performance of the system.