Wavelength division multiplexing(WDM) is in a great development recent years to meet the fast-grown demand for the communication capacity, and dense wavelength division multiplexing(DWDM) has been put into a successful commercial use. As a key device in a DWDM system, etching diffraction grating(EDG) is a planar waveguide DWDM device with a great potential. Although an EDG is harder to fabricate than an AWG(arrayed waveguide grating), it has a better spectral fineness and can easily implement more output channels. A design method for an EDG of flat input/output is presented. The scalar diffraction theory is used to verify the design, the numerical results show that the designed EDG has a good performance. A conventional EDG design is based on the Rowland circle on which the end points of the input and output waveguides are located. This configuration has a good focal property but needs the fabricating of the output channels to couple the light with output fibers. An EDG of flat-field input/outputs can also maintain a good linear dispersion and a perfect focused field as a conventional EDG.