In the paper, the effect of water injection and different air/fuel ratio (AFR) on the gasoline engine performance and emissions was investigated theoretically. A four cylinder SI engine model was built using GT-Power professional software. The gasoline fuel was injected directly to the cylinder and the water was injected into the intake manifold with different mass flow rate. The calculated engine outputs were: cylinder pressure and temperature, brake torque, brake power, mean effective pressure, thermal efficiency, carbon monoxide, hydrocarbon, and nitrogen oxide emissions. The results of simulations shows that the increased water mass flow rate resulted in improved engine performance and decreased emissions compared to neat gasoline fuel. However, we found that the excessive increase of the water amount inside the cylinder resulted in deteriorated engine performance and, consequently, poorly combustion efficiency.