The experiment of effect of CO2 dilution on combustion characteristics of H2-CO blended fuel by using a spark-ignition engine with fixed speed was carried out. The results showed that fast burning velocity of H2 offset its lower LHV, as a result, BMEP at each equivalent ratio kept almost constant. H2 substitution also benefited for thermal efficiency, engine stability, CO and NOx exhaust emissions. Suitable dilution hardly affected BMEP and thermal efficiency, but greatly reduced NOx emission. However, excess dilution induced to partial burn, which was bad for all these parameters of BMEP, thermal efficiency and CO emission.