On the basis of introducing the working principle of ultrahigh pressure common-rail system, the simulation model of ultrahigh pressure common-rail diesel engine of single cylinder was built by using GT-Power software, and the accuracy of this model was verified via experiment, then the effects of different fuel injection rate, saddle-shaped fuel injection rate coupled with different pilot injection fuel mass and different pilot injection advance angle on the power, economy, combustion and emission characteristics of ultrahigh pressure commonrail-diesel engine were analyzed by this model. The results show that the variable fuel injection rate can be realized by adjusting the opening time of the electric-controlled pressure amplifier solenoid valve and the injector solenoid valve in ultra-high pressure commonrail system. With the fuel injection rate varying from rectangular to saddle-shaped, the cylinder pressure, temperature and NO_x emissions of diesel engine decrease gradually, while the soot emissions rise gradually. The saddle-shaped fuel injection rate can make the diesel engine acquire the maximum power (torque) and the lowest fuel consumption rate. The saddle-shaped fuel injection rate coupled with lesser pilot injection fuel mass can make the diesel engine power, economy and combustion characteristics better, but it also leads to the rise of NO_x emissions. The saddle-shaped fuel injection rate coupled with larger pilot injection advance angle can improve the emission characteristics of diesel engine, but the effect is not obvious due to the limit of pilot injection fuel mass.