LLC resonant converter has gained popularity in a wide number of industrial applications due to its high efficiency and power density. However, disturbances such as input voltage fluctuations and parameter perturbations make the control of output voltage still a challenging problem. In order to reject the influence of disturbances on the stability of the output voltage, the linear active disturbance rejection control (LADRC) method is proposed in this paper. First, based on the equivalent circuit method, the analytical expression for control to output voltage is obtained and compared with the second-order transfer function obtained by the swept frequency method in the frequency domain. The second-order model that does not change the characteristics of the LLC resonant converter is used for analysis and design of the controller. Then, the linear extended observer (LESO) is designed to estimate the value of state variables and total disturbance. Finally, the simulation results demonstrated that LADRC can effectively control the stability of the output voltage compared with PI controller.