To better address instable utility frequency of thermal power units in the background of fast new energy development, a primary frequency modulation control strategy was proposed for a parameter-adaptive and active-disturbance-rejection flywheel-thermal power hybrid system. First, a primary frequency modulation model for hybrid system was established. On this basis, a power distribution method was adopted for hybrid frequency modulation system based on non-dominated sorting genetic algorithm III-variational mode decomposition. In this method, the adaptive non-dominated sorting genetic algorithm III was employed to optimize the key influencing parameters of penalty factor and modal number, thereby enhancing the variational modal decomposition and preventing overfitting and underfitting. Secondly, to solve the problems of slow response and large overshoot in traditional PID control for flywheel energy storage applications, an improved parameter-adaptive and active-disturbance-rejection controller was developed. After adequate consideration of varying requirements for speed and precision control at different stages of primary frequency regulation, the observer and gain parameters of the active-disturbance-rejection controller were dynamically adjusted based on frequency deviation and the rate of change, which improved the frequency modulation of the hybrid system. Finally, simulation verification was performed on the MATLAB/Simulink. The results show that the proposed control strategy reduces the maximum frequency deviation by 7.23% and 5.79% in the step disturbance and random disturbance tests, respectively.