Abstract: In order to study the propagation characteristics of two-dimension square lattice photonic crystal in THz rang, the band gap structure and density of photon states were calculated by plane-wave expansion method.It was found that the maximum photonic band gap generated when the f was 0.6362 and 0.7854 in the E and H polarization, and the maximum absolute photonic band gap generated when the f=0.7791;and large photonic band gap would generate by increasing difference inralue of dielectric constant;the photonic band structure was also shown by the photonic density of states.It turned out that this research provides theoretic basis for the development of THz devices.