For pilot fuel compression ignition natural gas (CING) engine, pre-injection strategy of pilot fuel is an important ways to improve engine performance, emissions and combustion. In this study, effects of pre-injection parameters on combustion and emissions performance were experimentally studied in a pilot diesel CING engine which was modified from a turbocharged six-cylinder diesel engine. The cylinder pressure, heart release rate (HRR), start of combustion (SOC), duration of combustion (DOC) and coefficient of variation (COVIMEP), as well as NOx and HC emissions were analyzed. The results indicate that early pre-injection mode leads to lower cylinder pressure and HRR due to decrease of combustion intensity, and thereby lower NOR emission is obtained. In contrast, closely pre-injection timing leads to largely strengthening in combustion which is not beneficial for improving NOR emission performance. Furthermore, effects of pre-injection quantity ratio and fuel injection pressure on combustion and emissions of early pre-injection operation modes (on pre-injection timing of 70 degrees CA BTDC) were analyzed. Increase of pre-injection quantity ratio leads to decreasing in ignition intensity which delayed SOC and slowed burning rate of in-cylinder mixture, and thereby leads to lower cylinder pressure and HRR. Due to slowing in in-cylinder combustion, combustion temperature decreased, and hence leads to lower NOR emission and higher HC emission. However, too high pre-injection quantity ratio leads to unstable ignition and even unstable combustion which is not beneficial for engine performance. Increasing fuel injection pressure leads to larger fraction of premixed quantity of injected fuel which increased ignition energy and leads to rapid combustion, and hence higher cylinder pressure and HRR were obtained. Due to the increase of burning rate of in-cylinder mixture, in-cylinder combustion temperature was increased, and hence leads to higher NOR emission and lower HC emission. (C) 2016 Elsevier Ltd. All rights reserved.