Abstract: Diesel engine has widely been used in the non-road mobile machinery industry, because of its considerable advantages in power, economy, and reliability. However, the working process of an diesel engine is rough, due mainly to the compression ignition operating mode Furthermore, the noise of the diesel engine cannot meet the requirements of living comfort. Therefore, it is highly necessary to reduce the noise of diesel engines. The noise of the internal combustion engine mainly includes aerodynamic, mechanical, and combustion noise. Combustion noise plays a leading role in the total noise of diesel engines, especially for direct injection diesel engines. Combustion noise is also closely related to the combustion process of internal combustion engines. The combustion noise is greater, as the combustion process is more intense in recent years. Fortunately, the noise can be reduced via the optimized parameters and tunable process. Therefore, the in-cylinder combustion process can also be optimized for the noise reduction of diesel engines. Since the pre-injection, exhaust gas recirculation (EGR), and turbocharging technologies cannot be widely used to reduce combustion noise for low power non-road diesel engines, due to the cost, structure, and application object. Therefore, the reasonable selection of parameters can make the matching of "fuel, air, and chamber" more perfect on the basis of existing low-power non-road diesel engine parts, thereby improving the combustion process in the cylinder. In this study, an optimization experiment was performed on the in-cylinder combustion of a diesel engine, in order to ensure the power performance, economy, NOx, and soot emissions of low power non-road machinery, while reducing the combustion noise in the diesel engine. A low-power non-road 4D29G31 diesel engine was used as an original engine. The optimization was made on the nozzle protrusion, nozzle hole number, nozzle hole diameter, and swirl ratio. The "fuel, air, and chamber" achieved the best matching state to improve the fuel-air mixing, the combustion in cylinder, and performance of diesel engine. The dynamic fuel supply advance angle was optimized to shorter the ignition delay time. The combustion rate and pressure oscillations were then be suppressed during the rapid combustion period. The maximum combustion pressure and the rising rate of the diesel engine under the rated condition were reduced by 18% and 44.9% after optimization, respectively, where the noise of the engine was reduced by 0.73 dB, compared with the original engine. The maximum combustion pressure and the rising rate of the diesel engine were reduced by 39% and 40% under the maximum torque condition, respectively, where the noise of the engine was reduced by 1.07 dB than before. It demonstrated that the combustion noise was reduced significantly in the whole diesel engine after optimization. At the same time, the economy and emission performance of diesel engines reached optimal in a low-power non-road diesel engine.