Cam profile is a key point to the design of high-pressure fuel system. It has a direct influence on the movement of the plunger, which affects the pressure characteristics of the pump. Based on the electronic controlled unit pump (EUP), the dynamic relationship between the cam profile and the pumping pressure was studied by means of experiment and simulation. It was found that within a certain period, the peak pressure was increased with the increase of injection duration using a constant speed cam profile, but the rising rate was decreased with the increase of the injection duration, and the peak pressure tended to be a constant value if the duration was long enough. With deceleration cam profile, the pump can reach approximately constant pumping pressure within common engine injection duration by adopting the dynamic performance in transitional period, in which the pressure was increased firstly and then decreased. In order to evaluate the quality of the constant pressure performance, an evaluation parameter was put forward, which was called constant pressure coefficient, and the influence of the acceleration rate in working section on the constant pressure coefficient was studied. Finally, in order to reach the constant pressure characteristic, a selection method of the optimum cam profile was put forward for different matching speeds. A supply cam profile was designed for a certain engine with a -0.013mm/(°CA)2 deceleration rate of working section and a good constant pressure characteristic (10~15°CA injection duration) and maximum cycle injection volume (438mm3) at engine speed of 1200r/min were obtained.