The reflective and transmissive phase shifts of photonic crystal Bragg mirror is studied,and the revelation is the reflective and transmissive phases all increase when the incident light frequency increases. In the vicinity of central frequency the phase shifts are linearly related to frequency. By applying the first order approximation of Taylor expansion the formulae of reflective and transmissive phase shifts are obtained in the vicinity of central frequency,and the change characteristics of phase shifts are well described. The characteristics of the resonant mode frequency of Bragg microcavity varying with the microcavity thickness is also studied,furthermore the resonant mode frequency is obtained as the first order approximation in the vicinity of central frequency.