A model of two-dimensional photonic band gap structures resonant cavity is proposed made by GaAs pillars in air with graphite lattice. Its TMy mode formant wavelength changing with stress and temperature is calculated by finite difference time domain method. The results show that no mater what direction for the stress acting on the resonant cavity, there will be a good linear characteristic between the formant wavelength and the stress, the sensitivity responding to stress is 0.013 69 nm/Mpa. Meanwhile, it is found that the sensitivities responding to stress along x and y direction are the same. There is a sectional linear characteristic between the main formant wavelength and temperature for this kind of photonic crystal resonant cavity, and the biggest sensitivity responding to temperature is 1.4 nm/℃. Furthermore, the results show that there is a make-and break key characteristic for the proposed resonant cavity.