Abstract: Due to its high thermal efficiency and excellent performance, diesel engine is widely used in non-road mobile machinery, such as engineering and agricultural industry, whose power ranges from a dozen to several hundred kilowatt. However, it also brings amounts of NOx and soot emissions. The problems caused by emissions from non-road diesel engine have widely drawn people’s attention. In recent years, Chinese government has also launched a series of stringent regulations to limit emissions from non-road engines, which can even be equivalent with the US EPA-IV, the most stringent emissions regulation in the whole world. In order to meet those harsh items of emissions regulation, all the engine manufacturers have to upgrade the products in a short time. At the same time, domestic and foreign internal combustion engine (ICE) experts have also carried out a lot of research related to the upgrading of non-road diesel engines. Small and medium power (< 75 kW) non-road diesel engines have innate advantages and wide applications in agricultural production. With the rapid development of after-treatment technology, such as diesel oxidation catalyst (DOC), diesel particulates filter (DPF), selective catalyst reduction (SCR), et al, the path to meet emissions regulations becomes more diverse. However, to meet the non-road stage IV emissions regulation, in addition to exhaust gas recirculation (EGR) and after-treatment technology, basic measurements (in-cylinder purification) must be used to reduce the raw emissions of engine. For the purpose of reducing raw emissions of the non-road heavy-duty diesel engine, experimental study was carried out on the in-cylinder combustion process of the diesel engine in this paper. CHD234V8 common rail direct injection diesel engine was used as a prototype. Through optimization design and matching of the diesel engine combustion chamber structure with fuel injection strategy, the quality of fuel and gas mixture in the cylinder was improved, the proportion of fuel impacting on the combustion chamber wall was optimized, and the maximum in-cylinder pressure was reduced. After the optimization of the diesel engine, the emission performance was greatly improved. Compared with the original engine, the specific fuel consumption at rated power and maximum torque conditions decreased 2.5% and 6.2%, and the smoke emission decreased 71.4% and 67.9%, respectively. CO, (HC+NOx), and particulate matters (PM) decreased 33.9%, 11.8%, and 73%, respectively. Emission performance of CHD234V8 diesel engine met the non-road stage III emissions regulation after optimization.