Abstract: Based on a 4F20 diesel engine with a mechanical fuel injection system, a high-pressure common rail diesel engine was developed to meet the national stage IV Emission Regulation. The optimization and matching of mechanical, combustion and after-treatment systems were conducted. For the development of the mechanical system, the ribs were added and a separated structure between the cylinder head bosses and the liner was utilized to increase the cylinder block stiffness and to reduce the liner distortion. The geometry of the upper water jacket in the cylinder block was designed to be circular, and its height was increased to match the TDC position of the first piston ring to improve the cooling effect of piston. The installing hole of for the glow plug was added on the base of three-hole layout, and then the coordinates of four holes were optimized. Degassing holes were added at corresponding position in the cylinder head gasket and cylinder head to eliminate dead flow regions and to enhance cooling effect; Coolant passages were set around the injector and above the intake, exhaust ports, and the local maximum temperature of cylinder head was reduced from 469.1K to 457.8K according to the optimized results. A new type of double row gear transmission system was designed which can run compactly and stably at low noise levels. For the optimization of the combustion system, the precise and flexible control of fuel injection timing and amount as well as split injection strategies were achieved by upgrading the mechanical fuel injection system to the BOSCH CRS2.0 electronically controlled high-pressure common rail fuel injection system. The injection pressure was improved significantly (the max pressure could reach up to 160MPa) as a result of the upgrade. The pre-injection can effectively improve the NOx emission about 30% at small and medium load, and low NOx and soot emissions were achieved while maintaining fuel efficiency after the introduction of post-injection at medium load. A big, open shallow combustion chamber was designed and the compression ratio was decreased from 19.7 to 17.5. Fuel spray circumferential distribution was planned according to the principle of equal arc length, also spray axial distribution was optimized, and the ratio of spray coverage volume to effective combustion volume was determined as 54.41%. For the air intake system, a radial-flow turbocharger with an exhaust bleeder valve was chosen to ensure that the compressor had enough safety margins from surge line at low speed, and it could work in highly efficient areas in the medium speed range. After applying the two-stage DOC system whose first and second volume fraction were 25% and 75%, the purification efficiency of CO, HC, SOF and PM reached 90%, 85%, 90% and 20% respectively. The developed diesel engine in this paper was equipped on an NHQ6492V3 SUV. The vehicle emissions test results showed that the emission of CO, NOx, THC+NOx and PM were decreased to 0.36, 0.259, 0.328 and 0.029g/km respectively, which were 20% lower than the national stage IV Emission Regulation limit. The comprehensive fuel consumption was 7.217L/(100 km) and it can meet the Chinese third stage regulation for passenger cars limit which will be implemented in 2015.