The fuel system of a diesel engine is a crucial component that significantly impacts the economic and technical performance, as well as the quality of exhaust emissions released into the environment. Improving fuel injection pressure, combined with changes in the fuel injection rules, the number of fuel injections in a cycle, and the geometric dimensions of the injectors, helps improve efficiency, reduce harmful emissions, and lower fuel consumption. This study evaluated the effect of the shape of the fuel injection characteristic at injection pressures up to 350 MPa, with the intake air pressure maintained at 0.25 MPa, using a 3D AVL Fire model. The results showed that at ultra-high fuel injection pressures, the amount of particulate matter from the diesel engine was very small, ranging from 0.0012 to 0.0062 g/kWh. By changing the distribution rules, the minimum amount of NOx was formed, corresponding to the case where the injection velocity was highest in the initial phase (case 1 and a value of 9.51 g/kWh) and gradually decreased in the latter phase of the injection process (case 5 and a value of 21.01 g/kWh). In addition, engine power decreased by 1.9% and fuel consumption increased by 1.8%. Due to inadequate fuel mixing between zones and fuel being mixed leaner than the combustion limit when increased injection pressure with a large injection volume in the initial phase. As a result, the maximum HC amounts increase by 4.8 times and the maximum CO amounts increase by 1.6 times compared to other injection cases.