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.