The basic method of H-infinity Optimum control theory was described and applied to the design of air fuel ratio controller in electronic fuel injection engine. The simulation model was established with the GT-Power software based on the physical model. In the case of outside disturbance and model uncertainty, the strategy of H-infinity Optimum was discussed and determined. The controller of H-infinity Optimum and PI was established with the Matlab/Simulink software. A real-time control system was established for electronic fuel injection engine through using the juncture between the GT-Power and Matlab/Simulink software. The simulation results have shown that the robust stability and disturbance rejection of H-infinity Optimum controller are better than those of PI controller, and H-infinity Optimum controller can improve air fuel ratio control accuracy.