Abstract: Since the global optimization seeking method used in the traditional multilayer design is very slow and difficult to get an ideal film system, an optimizing method based on the wellknown Frensnel equations and simplex optimization is used in the design of optical element of multilayer for soft Xray and Xray. The highest theoretical reflectivity of a soft Xray periodical multilayer at different wavelengthes is calculated, and the results are same as those calculated by the random search method, but the computing velocity of the simplex optimization is much faster than the random search method. The simplex method is also used in the optimizing design of the Xray supermirror, and the ideal result is obtained.