The limited capacity of the system inertia is pretty small, and it is highly susceptible to load fluctuations. Represented by digital radar is a kind of pulse load which power switches fast and in large range. It is pretty obvious with the frequency fluctuation and voltage fluctuation when the limited capacity of the system work with pulse load,and the dynamic process is complicated. In order to study the working characteristic effect on the limited capacity system of the pulse load .This paper built the simulation model about the limited capacity of the system with pulse load based on MATLAB/Simulink and defined the pulse under load calculation formula of electric characteristic of the limited capacity of the system and studied the effect law of pulse load on the limited capacity system. The results show that the model can be a real response characteristics of the pulse load and the effect law on the limited capacity system is different with the different load parameters.