The spark-ignited pre-chamber stratified combustion system is one of the most effective ways of expanding lean-burn ability and improving the performance of a natural gas engine. For these pre-chamber engines, the geometrical structure of orifices between the pre- and main chamber plays a significant role on the gas flow and flame propagation behaviors. The present study aims to investigate the effects of orifice number and diameter on combustion characteristics of a Shengdong T190 natural gas engine through CFD simulation. Various geometrical structures for the pre-chamber orifices were designed, offering variations in the number of orifices (4 to 8), and in the diameter of orifices (1.6mm to 2.9mm). A non-dimensional parameter β was employed to characterize the relative flow area of the orifices in the design.CFD simulations of combustion processes for these designs were carried out using a simplified chemical reaction kinetic mechanism for methane. Results show that, for a constant β value, the 6-orifice design can obtain the optimal results, while the design of excessive orifices leads to insufficient radial propagation of flames in the main chamber, and the design of less orifices leads to insufficient circumferential flames propagations in the main chamber. For a 6-orifice pre-chamber, a design of larger diameters leads to slower penetrating for the flame jets and insufficient radial flames propagations in the main chamber, and a design of relatively smaller orifice diameters leads to insufficient circumferential flames propagations in the main chamber. The optimal orifice diameter obtained in this study is 2mm, corresponding to a β value of 0.3. Consequently, it was confirmed that the optimal design is the 6-orifice with diameter of 2mm for the pre-chamber. This design achieves a 35.0% increase of indicated thermal efficiency and a 78.0% reduction of NOx emission compared to the prototype engine.