The dynamic response of optical resonant cavity has been theoretically studied,by taking resonant cavity as a linear and time invariant system and through the approach of Signal Processing.The mathematical description obtained could fit any form of light waves.From the perspective of Signal Processing,the traditional pulse and continues wave cavity ring-down spectroscopy could be considered as observation of impulse response and step response respectively.Furthermore,the cavity response to the light-wave linearly frequency chirped has been studied,in the condition of slow chirp.The Fourier analysis of light power passed through cavity has been made and an analytic description has been given,which reveals the fact that the signal spectrum of light power is related to the absorption spectrum of material inside cavity.Based on this theoretic analysis,a new approach of cavity enhanced absorption spectroscopy has been put forward,called Frequency Domain Cavity Ring-Down Spectroscopy.The result would facilitates the research of cavity enhanced spectroscopy techniques,as well others using resonant cavity,such as P-D-H laser frequency stabilization.