Taking a diesel engine equipped with a high pressure common rail system as the research object, using fluid dynamics simulation software FLUENT, the influence of the change of pilot injection parameters on engine emission performance and the variation of flow field in cylinder were studied. The results show that in a certain amount of pilot injection, with the pilot injection and the main injection interval angle increases, the pilot injection fuel combustion starts earlier, the time about the cylinder pressure reaching the peak is earlier, the emission of NO is lower, and the soot emissions increases first and then decreases. When fixed the interval angle of pilot injection and the main injection, with the increase of pilot injection quantities, the faster the cylinder pressure elevated, the more heat is released, the higher the NO emissions is, the emission of Soot increases first and then decreases.