In this paper a brief introduction of Etching diffraction grating (EDG) demultiplexing characteristic is presented.The traditional constant spacing design of the EDG parameters is not suitable for the planar waveguide.Two design schemes:1 stigma method and 2 stigma method,is analyzed in detail in this paper.The grating points determined respectively by the two methods are evaluated by the light path error.The simulating results show that the grating structure designed by 2 stigma method is better than that by 1 stigma method.