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In this paper, a small-signal model of a kind of peak current-mode pulse-width-modulation (PWM) control buck converter is presented which uses the circuit state average approach and combines with the consideration of influence by load change, and mutual effect between voltage feedback loop and current feedback loop. Based on the small-signal model, this paper proposes a novel method of systematic stability design. The principle is, firstly, just considering the stability design of current inner loop by cutting off the voltage outer loop; when the current inner loop compensation succeeds; we close the inner loop and complete the design of outer loop compensation. Consequently the stability design of the whole systematic circuit completes. This paper also gives a brief discuss on the issue of input immunity and output immunity. Finally, by using Matlab Simulink, based on system equivalent model which is established in this paper, and Hsipce, based on transistor-level circuit, this paper works out the system transient response simulation. Results from the simulation show that the system small-signal model proposed in this paper is reasonable, and the system-level design based on the model is feasible. Also, it is capable to enhance the system stability and has good input and output immunity.