Natural gas is one of the cleanest alternative fuels due to the low cost, wide availability worldwide, and other advantages, e.g., high knock resistance and low carbon content. Ignition quality is a key factor influencing the combustion performance. In this study, the effect of prechamber geometry on the ignition process and flame propagation has been studied with varied initial mixture temperature and equivalence ratio in comparison with a conventional spark plug in a constant volume bomb. The prechamber with orifices in different shapes (circular and slit) and diameters were used. Schlieren imaging was adopted to acquire the flame propagation. The results show that under the same cross-section area, the slit prechamber can accelerate the flame burning rate. Under the experimental conditions mentioned in this paper, as for the prechamber with orifices in different shapes, the larger the cross-section is, the better the natural gas has been burned. Under the same initial temperature, the combustion characteristics under different excess air coefficients are different, both in the prechamber with slit and the prechamber circular hole.