The effect of nozzle diameter, injection duration and injection advance angle on flammable mixture and dense area center of single orifice CNG engine was studied by numerical simulation. The model was verified by the comparison of the acquired length and width of jet flow between test and simulation with the schlieren method. The results showed that the small orifice led to the dense area center moving to combustion chamber wall and the rapid formation of mixture after squeezing. The large orifice led to a lower kinetic energy after squeezing that caused the poor mixing quality and made the dense area center close to chamber wall. The increase of injection duration increased the mixing velocity and flammable mixture quantity, but the dense center was still close to chamber wall. The excessivelyextending injection duration would change the shape of jet flow, which influenced the formation of flammable mixture. The smaller advance angle enhanced the kinetic energy of gas after squeeziing which increased the mixing velocity. The flammable mixture amount was influenced by mixing duration and the dense area center was easily influenced by the motion of gas in the eddy. Optimizing the nozzle diameter, injection duration and injection advance angle could promote the formation of homogeneous mixture and the latter two parameters were more beneficial to improve the homogeneous mixture.