Aiming at the problem that the disturbance generated by the DC / DC converter using predictive function control PFC has a greater impact on the response speed of the converter during load switching it is proposed to optimize the control effect of the predictive function control algorithm by designing the observer to observe the disturbance. Compared with other methods to improve the system immunity the advantage of improving the converter immunity by designing an observer is to achieve simplicity and good control effect after optimization. The Luenberger observer was designed to observe the disturbance generated during load switching feed the observation value back to the prediction function control algorithm for optimization analysis and adjust the objective function in combination with the simulation results. The optimized objective function can better respond to the disturbance and reach a steady state as soon as possible. Finally the optimized control algorithm was compared with the PI double closed-loop control method and the results show that the optimized control algorithm has better dynamic performance than the dual-PI control method after being disturbed which achieves the goal of improving the dynamic performance of the system during load switching and enhances the antiinterference ability of the DC / DC converter. The optimized algorithm only takes into account the disturbance during load switching and does not take into account the disturbance of other aspects of the system and the ability of the DC / DC converter to resist disturbance can be further improved by designing the observer of the entire system.