To address the issue of output voltage disturbance in Buck converters under complex environments with load fluctuations, a composite adaptive prescribed performance control scheme is proposed to enhance control effectiveness. Initially, an adaptive law is utilized to predict and estimate the nonlinear function containing the load term within the model. Concurrently, a parallel estimation model is constructed during the adaptive law update process to acquire prediction errors, which are then integrated with tracking errors to design an adaptive parameter update law. Subsequently, a generalized proportional-integral observer is employed to estimate the remaining uncertainties and external disturbances, which are compensated for within the control law. Finally, combining command-filtering backstepping control and specified-time prescribed performance control techniques, a composite adaptive prescribed performance control scheme for Buck converters is proposed. The presented scheme ensures high-precision prediction of load fluctuations, preventing output voltage from exceeding the prescribed function range during sudden events, and also demonstrates the signal convergence within the closed-loop control system. Experimental results indicate that the composite adaptive prescribed performance control, when faced with a sudden reduction in load, limits the system’s maximum voltage deviation to 0.376 V, a 78.7% decrease compared to the traditional adaptive backstepping control’s 1.773 V, thereby validating the effectiveness and superiority of the proposed scheme.