Aiming at solving the difficulties in hydrogen on-board storage and carriage, a self-designed system was designed in an SI gasoline engine to recover the exhaust heat and produce hydrogen by the ethanol steam reforming. The engine intake manifolds were specially modified so that the produced syngas could be injected and mixed with gasoline simultaneously. During the experiment, the engine was kept at 1800r/min and a manifolds absolute pressure of 61.5kPa. The combustion and emissions performance of the 2.5% syngas-enriched gasoline engine was compared with that of the original engine under various excess air ratios. The experimental results demonstrated that the enhancement of excess air ratio could improve the hydrogen volume fraction whereas reduce the CO concentration in the syngas. The blending of syngas was beneficial for accelerating the flame development and propagation, elevating the indicated thermal efficiency and reducing the HC emissions. Meanwhile, the CO and Nox emissions were slightly increased after the syngas addition.