An experimental study was made to investigate the effect of residual gas on the combustion characteristics and flame propagation of butane-air mixtures in a constant-volume combustion chamber. The combustion process and flame propagation are studied under different ratios of residual gas and various equivalence ratios in the combustion chamber. The effects of the residual gas ratio on the combustion pressure, heat release rate, burned fraction, and flame propagation phenomena were studied in detail. The experimental apparatus consists of a constant-volume combustion chamber, a premixture chamber, a data acquisition system, and a laser Schlieren system with a high-speed camera. With an increase of the residual gas ratio in the combustion chamber, the combustion pressure and the rate of heat release decrease and the burning period of the fuel-air mixture is reduced by the increase of residual gas. The effects of residual gas on the combustion characteristics and flame propagation speed are dependent on the amount of residual gas. In the case of a higher residual gas ratio (more than 10%) in the chamber, the combustion pressure and heat release are steeply decreased. It is confirmed that residual gas in the combustion chamber lowered the rate of heat release as a result of the decrease of combustion temperature. The flame propagation speed decreases with the increase of residual gas in the combustion chamber.