This paper presented the different mass motion and the mechanism, analyzed the effect of the mass motion on the combust process of the engine. Combustion chambers with different structure and size that fit the combustion of the lean burn CNG engine were given and the effect of the chambers with different structure on the performance of CNG engine was studied through numerical study and experiment. The result shows that the combustion of lean burn engine with high turbulence chamber is fast, provides more power and economic performance, but high NOx emissions.