Intake air modulation in combination with electronic fuel injection to control air fuel ratio is one of the important measures to further improve the performances of gasoline engines.The paper redisigns some of the structures of a single cylinder gasoline engine,designs an electronic throttle system for intake air modulation,determines the control algorithm for air fuel ratio,develops an air-fuel ratio control system to adjust the amounts of inlet air and injected fuel.From dynamometer tests,it is shown that the performance,fuel economy and exhaust emission of the engine are improved obviously.