It is revealed in the present paper that the reflectance around the defect mode of one-dimensional defective photonic crystal (PC) in an asymmetric structure approaches to 1, while the phase-shift depends on the number of the coupled-defect layers, i.e., the phase shift is 2π for every sub-peak of the defect mode. When the defect layer is a temperature sensitive material, very small change of temperature will cause a significant phase change. Secondly, it is demonstrated that the relationship of phase and temperature has a linear range. According to the above characteristics, a highly sensitive temperature sensor is designed based on the phase property of photonic crystal. Moreover, this principle of PC phase sensing can be extended to study other sensors, such as the two-dimensional PC, which is suitable for optical integration.