Future high-power-density engines require high level of intake boost. However, the effects of boosting on mixing, combustion and emissions in existing studies are inconsistent. In this paper, the mixing, combustion and emission characteristics with intake pressures of 100–400 kPa at low, medium and high loads are studied. The results show that the increase of intake pressures is conducive to the enhancement of air entrainment, while the air utilization ratios are reduced, thus requiring injection pressure to be optimized to effectively improve the mixing. For the intake pressures of 100 kPa, the average chemical reaction path is low-temperature reaction route, while the path of higher intake pressures is dominated by high-temperature pyrolysis. For soot emissions, when the equivalence ratio is lower than 0.175, the oxygen in the cylinder is sufficient, so the effect of temperature decrease is more significant, which leads to the increase of soot emissions with the increase of intake pressures. Otherwise, the effect of increasing oxygen concentration is more significant, so soot decreases accordingly. When the peak of global temperature is lower than 1800 K, the effect of the increase of oxygen concentration is more dominant, so the NOx emission increases with the increase of intake pressures. Otherwise, the rule of NOx emissions is consistent with temperature changes.