Three-dimensional computation of spray and combustion in 16V240ZJ diesel engine was performed by CFD numerical simulation tool FIRE. Because of the unceasing change of the temperature and the mixture in the cylinder, traditional performance-prediction method can only supply the limited information. The appearance of numerical simulation brought a bright future for the design and development of the new diesel engine. Combustion is an extremely complex process ,involving flow field, fuel injection and various combustion reactions. By calculus simulation, we have a thorough understanding of the combustion process of 16V240ZJ diesel engine. Through the flow field and concentrative analysis in the cylinder, the effect of advance angle of fuel on combustion process and formation of NOx and soot was investigated. The results show that the advance angle of fuel has a great effect on spray atomization and fuel-air mixture, and consequently influences the combustion and emission. By simulating , we can get the optimal advance angle of fuel for 16V240ZJ diesel engine is 15°BTDC, namely 345°CA, at which, the emissions are fewer ,and the efficiency is better .The CFD simulation has a direct value for optimizing the chamber’s structure and improving the combustion system.