Water window is a special wave band for microscopy in soft X-ray range, which makes the research in water window polarization great significance. With Fresnel formula, the maximum reflectivity is calculated for different materials at corresponding wavelength in water window, the influence of the multilayers' periods and surface roughness on the element performance is analysed. Three W/B4C multilayers are produced for polarizing elements in special wavelengths 2.40 nm, 3.00 nm and 4.30 nm based on ultrahigh vacuum magnetron sputtering equipment, the periodic thickness of elements by X-ray diffraction instrument is measured. The results with little windage prove practical. The work is essential in the study of polarizing elements of reflection multilayeres in water window which provides the theory and the suitable parameters for mutilayeres fabrication.