Abstract: To address the issues of poor dynamic performance and weak disturbance rejection capability in LLC resonant converters under traditional control methods, a fractional linear order active disturbance rejection controller (FOLADRC) was designed and applied to the closed-loop control system of a half-bridge LLC resonant converter. Initially, the small-signal model of the LLC resonant converter was obtained through the extended describing function method, and the system transfer function was obtained through reduction in Matlab. Subsequently, based on the framework of linear active disturbance rejection controller (LADRC), the tracking differentiator module responsible for phase lag in LADRC was eliminated to mitigate dynamic response delays. Furthermore, the error feedback control law in LADRC was replaced with a PD^μ controller, thereby incorporating the differential order μ from the FOPID controller to regulate system dynamic performance. The designed FOLADRC synergistically combined the active disturbance characteristics of LADRC and the dynamic advantages of FOPID controller. The transfer function of the FOLADRC was analytically derived, and the system’s Bode plot was generated using Matlab. The stability margin of the controller was validated through frequency-domain analysis. Finally, a 300 W experimental prototype was built to compare the performance of PID, LADRC, and FOLADRC control strategies. The results show that in the positive and negative load step response conditions, compared with the PID controller, FOLADRC shortens the transient adjustment time by 47.37% and 60% respectively. Compared with the LADRC, the transient adjustment time is shortened by 20% and 31.03% respectively, significantly improving the system’s dynamic response speed and anti-interference capability. This study provides an effective solution for optimizing the performance of LLC resonant converters through the integration of fractional-order control and active disturbance rejection theory.