The system classification and mechanism research of water injection in the gasoline engine were reviewed and then the water injection strategy of gasoline engine was discussed and summarized. The water injection timing and the position of water injector were thought to be important factors that would affect whether the injected water in the intake port could enter the cylinder and cause the heat absorption by evaporation successfully or not. The water injection timing and pressure were important optimization parameters that would affect the incylinder cooling effect for the incylinder water injection technology. The research fruits of water injection such as suppressing knock, eliminating fuel enrichment and expanding the compression ignition operation area were mainly introduced. In the end, the limitations of largescale application of water injection technology in the future were discussed by comparing it with traditional EGR technology.