Under the context of increasing rail pressure and reducing orifice diameter for highly intesified diesel engines, the combustion processes of diesel engines with different injection hole diameters were tested and studied under three operating conditions, the combustion and emission results were compared and analyzed, and their working mechanisms were given through the simulation research. The research results show that both the fuel consumption rate and the soot decrease, the fuel consumption rate reduces by up to 2.5% and the soot reduces by 63.9% as the hole diameter decreases from 0.30 mm to 0.25 mm during the test process of the single-cylinder diesel engine. However, when the hole diameter decreases from 0.25 mm to 0.24 mm, the in-cylinder combustion process shows a consistent deterioration trend. The three-dimensional in-cylinder simulation results under the calibration operating conditions show that the matching of hole diameter to operating condition has a range of application, and the combustion shows a deterioration trend for the hole diameter of 0.24 mm and 0.30 mm beyond this range, while for the hole diameter of 0.25 mm, the fuel and gas mixes well, and the thermal efficiency reaches the highest, therefore the optimal combustion is achieved.