In the designing of a grazing angle multi layer mirror at the wavelength 1.03 nm in soft x ray regin,a modified method is presented in which the effect of root mean square roughness of substrate and interface to reflectance is considered and discussed.The mathematical describing for rough interface is from the scattering theory by D.G.Sterns.The design result shows that the substrate roughness should be smaller than 0.56 nm in order to fabricate the grazing angle multi layer mirror whose reflectance is greater than 10 %.According to the result above,we chose a finish machining substrate whose roughness is 0.50 nm and fabricated a multi layer mirror measured reflectance is 10 %,which is consistent to the modified method.