A 3-D simulation model including the inlet and the cylinder was built to study the backfire mechanism of PFI hydrogen internal combustion engines. A method controlling the backfire was put forward through optimizing the hydrogen injection angle. The low concentrated mixture gas was flow into the cylinder first after the intake valve opening, which can then lower the temperature of the exhaust gas and hot point inside the chamber. Based on the simulation model, the hydrogen injection angle influenced on the concentration and temperature of the mixture gas with the engine at different working speeds and conditions was investigated. The optimized hydrogen injection angle causing no backfire in all working conditions were found. It was proved by the experiment that the hydrogen internal combustion engine with the optimized hydrogen injection angle worked normally without backfire when the equivalent fuel air ratio of mixture gas reaching or even exceeding the stoichiometric fuel-air ratio within the speed range.