Abstract: A system and a method for controlling water injection of an internal combustion engine are disclosed. A proper amount of water is injected into the internal combustion engine serving as a core carrier to enlarge the contact area of the gasoline fuel with the combustion-supporting air, and reduce the overall temperature of the combustion chamber. The system monitors the revolving rate of the engine to control the water intake timing. Giving a car engine as an example, when the driver releases the accelerator significantly, no water is required to enter the combustion chamber of the engine practically. Therefore, the control system stops to inject water into the car engine once it detects a quick drop of the revolving rate of the engine.