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 1.6L gasoline engine at a speed of 1800r/min, a manifolds absolute pressure of 61.5kPa 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.