Abstract: Experimental study on the effects of injection timing, like pre-injection timing and main injection timing, on emissions and combustion process was carried out in low speed and low load conditions of a heavy-duty diesel engine. The results show that the NO_x and Soot emissions gradually increased, HC and CO emissions decreased and the thermal efficiency gradually increased as the pre-injection timing postponed to the TDC while the main injection timing, the fuel ratio of the pre-injection and the main injection and the EGR rate were under the same conditions. And when the pre-injection timing, the fuel ratio of the pre-injection and the main injection and the EGR rate were under the same conditions, the NO_x emissions would decrease, Soot emissions would first increase and then decrease, HC and CO emissions would increase and the effective thermal efficiency would decrease gradually as the main injection timing was postponed.