SI engine experiments were carried out by using hydrogen at equivalent ratio of 0.8, 0.6, and 0.4 and by using gasoline at stoichiometric condition, respectively. Simulation models for gasoline and hydrogen were built based on AVL Boost and calibrated experimentally. The simulation study shows that hydrogen can increase burning rate of hydrogen-air mixture, and shorten combustion duration markedly. As a result, cylinder pressure and thermal efficiency increase accordingly. Hydrogen engine can operate at the equivalence ratio of near 0.4 smoothly and the performance of SI engine is greatly improved. 