Nowadays, the use of ammonia as a green fuel for internal combustion engines has attracted wide attention. The diesel/ammonia dual direct injection mode has shown great potential, but there is still a lack of basic research on injection strategies for this mode. In this study, the combustion and emission characteristics of diesel/ammonia dual direct injection mode were investigated using a rapid compression and expansion machine (RCEM) combined with CONVERGE software_v3.0. The research focuses on the effects of two injection strategies, including ammonia injection pressure, the ammonia injector nozzle hole diameter, and the compression ratio. The results indicate that minor increases in ammonia injection pressure have negligible impacts on emissions with the same nozzle hole diameter. Increasing the nozzle hole diameter significantly reduces unburned ammonia emissions while increasing HC and N2O emissions. Increasing the compression ratio enhances diesel combustion but does not significantly affect ammonia combustion. Considering the ammonia energy substitution rate and the combustion performance of the actual engine, a high ammonia injection pressure and compression ratio are necessary for engine applications, while an appropriate ammonia orifice diameter is required to meet the emission performance.