The band structure of two-dimensional photonic crystals consisting of the silicon (Si) dielectric cylinder square lattices is calculated by using plane-wave expansion method for the TM mode. Photonic crystal microcavity structure is designed. The defect-state field of the microcavity is simulated by finite-difference time-domain (FDTD) method,so the mode field distribution of defects is obtained. The thermal expansion and thermal-optic effect of silicon is taken into account,the resonant wavelengths of the microcavity are also calculated by FDTD method under different temperature. The results indicate that the resonant wavelength increases linearly while the temperature rising. The wavelength shift is 6.7 pm/℃. This characteristic of photonic crystal microcavity can be used for temperature sensing,which has a certain degree of practical significance.