Combustion experiments of CH4,H2 and CO were carried out in a spark-ignition engine under 1500r/min engine speed, and the combustion and emission characteristics were analyzed. The results showed that H2 had the fastest burning velocity, the highest thermal efficiency and the lowest NOx emission. Comparing with CO, CH4 had higher thermal efficiency, lower NOx emission and poorer lean burn ability. In practical application, it had potential to improve lean burn limit and thermal efficiency of biogas by mixing with H2.