Abstract: A gasoline engine with 2.0L port injection was modified to a port injection hydrogen engine. The mechanism of how hydrogen injection timing had influence on the intake pressure wave was studied by engine testing. Improving the hydrogen engine airflow by utilizing optimized hydrogen injection was proposed. The test results showed that the manifold pressure increased and pressure wave amplitude was enhanced by hydrogen injection. The manifold pressure was not only influenced by its own injection, but also driven by other cylinders. More airflow could be achieved when hydrogen was injected between bottom dead center to intake valve closed, or 180℃A away from above range at low speed. Maximum airflow for high speed only can be achieved when hydrogen was injected between bottom dead center to intake valve closed. There would be 4.4% (low speed) to 7.3% (high speed) better airflow by utilizing optimized hydrogen injection.