The basic methods of H-infinity control theory and model of standard H-infinity design problem were described. The H-infinity robust control theory was applied to the design process of CNG engine idle speed control. The dynamic model of CNG engine was established. The diagram of engine uncertainty was stated. A controller of CNG engine idle speed was successfully developed based on H-infinity control theory and Matlab robust toolbox. The simulations and experiments about the CNG engine were done and the results show that the H-infinity controller possesses good robust features and can improve the stability of CNG engine idling operation.