A new modeling and the analysis methods were proposed for Buck converter in continuous current mode(CCM)in this paper.The models of the zero-state responses and the zero-input responses for Buck converter were established.The delay characteristic of the responses was found and was expressed by using a time -sequence.An accurate closed-form analytical solution(symbol form)was obtained in this paper.The analytical solution expresses the relation between the duty and output voltage of converter and is a decoupled form between the inductor current and capacitor voltage.The analytical solution can express the transient state

steady state and ripple of the converter and the switching frequency were not limited.A Soft-Start strategy for Buck converter based on digital control was realized by using the method proposed and was more precise and simpler.The simulation experiments showed the validity of the method proposed.