In the future, ammonia is expected to become a carbon-free fuel for internal combustion engines. However, the flammability of ammonia is poorer compared to conventional fuels such as gasoline and diesel fuel. Pre-chamber jet ignition may be an effective way to ensure stable ignition and enhance the combustion of ammonia. In this paper, the effects of pre-chamber geometric parameters, including volume and orifice diameter, on the jet ignition and combustion processes were studied using visualization methods, combined with pressure acquisition. The results showed that ignition energy increased and the jet duration was prolonged with the increase in pre-chamber volume, resulting in a higher maximum pressure and pressure rise rate in the main chamber. The jet characteristics of a larger volume pre-chamber exhibited higher stability when the ambient parameters were changed. The smaller volume pre-chamber showed the superiority of a shorter flame propagation distance inside the pre-chamber, which advanced the timing of the jet appearance and shortened the ignition delay when the flammability of the pre-mixture was adequate. The larger pre-chamber orifice diameter caused an earlier jet ignition timing, shorter ignition delay, and higher ignition location. The jet duration for the pre-chamber with a smaller orifice was longer, which was beneficial for increasing the pressure rise rate in the main chamber. Too small a pre-chamber orifice led to ignition failure in the main chamber.