To improve the dynamic performance of the two-stage AC/DC converter and enhance the system’s ability to resist disturbances, an improved active disturbance rejection control (ADRC) method for the two-stage converter is proposed. Firstly, small-signal analyses of the pre-stage power factor corrector (PFC) circuit and the post-stage inductor-inductor-capacitor (LLC) resonant converter circuit are carried out based on the switching cycle average method and the extended describing function method, respectively, and a mathematical model of the circuit is established. Then, by considering the double-frequency ripple as an internal disturbance, a disturbance rejection law is incorporated into the ADRC. This allows the observer to quickly predict the extended state and further study the influences of different disturbance rejection laws on the system’s dynamic performance and resistance to disturbances. A method for designing the controller’s parameters is also given. Finally, simulation experiments are conducted to test the performance of the controller. The experimental results show that, compared with the traditional ADRC and proportional integral (PI) controllers, the improved ADRC can improve the overshoot and regulation time of the output voltage when the load is abrupt under lower current harmonics, enhancing the dynamic performance of the system.