To address the poor transient response and large DC bus voltage fluctuations of the power conversion system (PCS) under power step changes, a fractional-order complementary sliding mode control (FOCSMC) strategy based on an improved high-gain observer (HGO) is proposed. The proposed method is applied to the power control loop of the DC-AC converter and the DC bus voltage control loop of the DC-DC converter. Firstly, a system model of the PCS is established, in which internal and external disturbances as well as coupling effects are aggregated into a lumped disturbance, leading to an active disturbance rejection formulation. To enhance the observation capability and mitigate the adverse effects of high-frequency measurement noise, a high-gain coefficient is incorporated into the conventional linear extended state observer (LESO) and a phase-lag compensation term is introduced in the disturbance estimation path. Frequency-domain analysis verifies that the proposed HGO achieves superior observation accuracy and disturbance rejection performance over the traditional LESO. Secondly, the error feedback control law is designed based on FOCSMC, and a double-power reaching law is introduced to accelerate error convergence and alleviate chattering. The stability of the overall control scheme is rigorously proved via Lyapunov stability theory. Finally, experimental results demonstrate that the proposed control strategy exhibits significantly better transient response and lower DC bus voltage ripple than conventional proportional-integral (PI) control and active disturbance rejection control (ADRC).