The spark-ignited pre-chamber stratified combustion system is one of the most effective way to expand lean-burn ability and improve the performance of a natural gas engine. For these pre-chamber engine, the geometrical structure of orifices between the pre- and main chamber play a significant role on the gas flow and flame propagation behaviors. The present study aims at investigating the effects of the orifice number and diameter on the combustion characteristics for a Shengdong T190 engine. Various geometrical structure for pre-chamber orifices were designed, offering variations in the number of orifices (2 to 6), and in the diameter of orifices (1.66mm to 4.98mm). 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 model for methane. Results show that, for a constant orifice number, the indicated power initially increases then decreases with the increase of β, while the NOx emission gets the opposite trend. The highest indicated power is achieved when the β value is 0.4. Lower or higher value of the β will cause the flame to spread towards bottom or top parts of the main chamber, respectively. For the constant β value of 0.4, the indicated power initially increases then decreases as orifice number varying from 2 to 6, and the design of 4 orifices obtained the highest indicated power. Design of fewer and more orifices will lead to insufficient circumferential and radial propagation of the flames in the main chamber, respectively. Finally, it is found the optimized geometrical structure of the orifices is a combination of 4 orifices with 3.0 mm diameter. This design achieve a 4.0% increase of indicated power and a 54.6% reduction of NOx emission compared to the prototype engine.