Phase spectrum of fiber Bragg grating (FBG) has a major effect on spectral characteristics of the constructed Fabry-Pérot (F-P) cavity. By the analysis of spectral characteristics of an FBG, especially its phase spectrum, the linear phase spectrum of a weak grating is derived. Furthermore, the three-segment linear approximation is proposed for strong gratings with a concise expression. Then, based on the deduced analytic expressions of phase spectra, spectral characteristics of FBG based Fabry-Pérot (FBG-FP) cavities are investigated by the conception of effective cavity in comparison with general F-P cavities. The fitting method, periodic method and Fourier transform method for FBG-FP cavity length estimation are discussed. A scheme of spectral measurement of FBG-FP cavities is designed, which obtains spectral curves with a high accuracy. The above three methods for cavity length estimation are compared mutually with tested spectra, and theoretical analyses are validated.