The mechanism of photonic crystal pressure sensor was described in this paper, after an examination of the relationship of the pressure on a one-dimensional photonic crystal and its photonic band gap (PBG) property. The results show that there is a simple relationship between the pressure and the stop wave length. This makes it possible to know the load by studying the PBG property or to change the PBG property by loading; the loads can be mechanical loads or thermal loads. Since the structure scale is similar to the wave length, it is possible to realize a series of sensors with very high exactness.