The phase diagrams are used to ascertain positions of 1 D photonic crystal band gaps. The positions and characteristics of the photonic crystal band gaps can be expediently depicted by using patulous phase diagrams. The band-gap width is largest when the photonic crystal is composed of alternately arranged quarter wave stacks. Increasing the ratio of refractive indices can enlarge the band-gap width. For a conventional photonic crystal,the central position of every band gap will depart from the integral times of the central frequency if the optical thickness of both media in periodic unit is not equal. Furthermore,the transmittance in the central region of the band gap is studied,and the formula of transmittance near the central frequency with the first approximation are deduced. The spectrum line width and quality factor of the Fabry-Perot cavity are qualitatively discussed.