Engine experiments have shown that with high-pressure multiple injections (two or more injection pulses per power cycle), the soot-NOx trade-off curves of a diesel engine can be shifted closer to the origin than those with the conventional single-pulse injections, reducing both soot and NOx emissions significantly. In order to understand the mechanism of emissions reduction, multidimensional computations were carried out for a heavy-duty diesel engine with multiple injections. Different injection schemes were considered, and the predicted cylinder pressure, heat release rate and soot and NOx emissions were compared with measured data. Excellent agreements between predictions and measurements were achieved after improvements in the models were made. The improvements include using a RNG k-ε turbulence model, adopting a new wall heat transfer model and introducing the nozzle discharge coefficient to account for the contraction of fuel jet at the nozzle exit. The present computations confirm that split injection allows significant soot reduction with out a NOx penalty. Based on the computations, it is found that multiple injections have a similar NOx reduction mechanism as single injections with retarded injection timings. Regarding soot reduction, it is shown that reduced soot formation is due to the fact that the soot producing rich regions at the spray tip are not replenished when the injection is terminated and then restarted. With split injections, the subsequently injected fuel burns rapidly and does not contribute significantly to soot production. The present work also demonstrates the usefulness of multidimensional modeling of diesel combustion to reveal combustion mechanisms and to provide design insights for low emission engines.