A combustion system model of CNG-DI engine which was modified from a 2.0L diesel engine was built with software FIRE,the turbulence combustion model of CNG-DI engine was verified with the experimental result of CNG-DI optical engine,on the condition of unchanged 4 valves structure of original engine and the jet location,the spark plug was arranged between the two exhaust valves,the effect of different combustion chamber shapes on the microscopic physical fields such as turbulence characteristic in cylinder was simulated and studied,based on which the effect of combustion chamber shapes on the formation and combustion of mixture and the formation rule of NO was researched.The result shows that he combustion shape has a tremendous influence on the turbulence characteristic of cylinder and distribution characteristics of concentration fields especially besides the spark plug,which plays a decisive role on the whole process of combustion; Not only the production of NO can be inhibited and the combustion propagation speed which is in favour of lean burn can be controlled effectively when the straight mouth combustion chamber was used at the bottom of which has a proper embossment.The amount of NO is closely related to the size of the reaction area and the duration of reaction besides the reaction rate of NO when the CNG fuel is injected into cylinder directly and burned thin,which can be controlled by the rational design of the structure of combustion chamber.