Cavity ring-down (CRD) spectrum of fiber Bragg grating Fabry-Perot (FBG-FP) cavity is researched theoretically. The expression of output light intensity of FBG-FP cavity is derived, and the effect of the power reflectance of FBG-FP cavity on CRD spectrum is discussed. Numerical results show that the output performance of FBG-FP cavity includes three phases: build-up, stability and ring-down phase. The light intensity in the cavity and CRD time of cavity are proportional to the power reflectance of the FBG-FP cavity. Finally, the response of FBG-FP cavity to injected laser pulse is experimentally demonstrated. Experimental results are consistent with the theoretical analysis.