In order to obtain the effect law and genesis mechanism of fuel injection advance angle on the performance of diesel engine under the adjustable fuel injection law conditions, on the basis of the realization of adjustable fuel injection law injection, the working process calculation model of diesel engine was established by Fire software, and the main parameters of model were calibrated through experiments, then the performance of diesel engine with the fuel injection advance angle under saddle-shaped fuel injection law was analyzed with the model. The results show that with the delay of pressurization time, the fuel injection law changes from rectangle to slope and then to saddle-shaped, which realizes the flexible and adjustable fuel injection law. Under the condition of saddle-shaped fuel injection law, there is an optimal fuel injection advance angle to optimize the power and economy of diesel engine. At the same time, with the increase of fuel injection advance angle, the in-cylinder pressure, in-cylinder temperature and heat release rate of diesel engine increase gradually, and the time of reaching their peak values all move forward, while the NO_x emissions and soot emissions show an increasing and decreasing trend respectively.