The　onboard　fuel　reforming　could　not　only　recovery　the　exhaust　heat　but　also　produce　syngas　for　the　mixed　combustion　in　cylinders.　An　experimental　study　was　carried　out　on　a　four-cylinder　1.6　L　gasoline　engine　at　a　speed　of　1800　r/min,　a　manifolds　absolute　pressure　of　61.5　kPa　and　the　maximum　brake　torque　for　the　spark　timing　to　explore　the　effect　of　syngas　addition　on　the　engine　combustion　and　emission　performances　at　the　stoichiometric　condition.　The　experimental　results　demonstrated　that　the　hydrogen　volume　faction　in　syngas　increased　with　the　increase　of　syngas　addition　faction.　The　fuel　consumption　rate　was　decreased　and　the　indicated　thermal　efficiency　was　improved.　HC,　NOx　and　CO2　emissions　were　reduced　whereas　CO　emission　was　increased　after　the　syngas　enrichment　at　the　stoichiometric　condition.