A two-dimension photonic crystal with triangular air hole was designed in silicon backgroud material.Plane-wave expansion method is used in study of numerical value of energy band structure of 2D photonic crystal.The band gaps were investigated by changing the side length of triangular air hole,lattice period and rotation angle of elements.Research results show that the proper choice of the side length of triangular air hole,lattice period and rotation angle will make the photonic crystals have absolute band gaps.A large absolute band gap is found by optimizing the parameters.