Abstract: Based on the combination of thermodynamic calculations and experimental verification, the influences of Cr, W, C and Mn on the microstructure and matrix phases of high-chromium reduced-activation ferritic/martensitic (F/M) steels were systematically studied. On this basis, the microstructure and chemical compositions of steels were optimized to meet the expectations of supercritical water system for cladding material performance requirements. Cr is the most important element which determines both the ferrite amount in the steel and the Cr solution amount in the austenite, while W and C also have significant impact on whether the ferrite appears in the matrix, though the influence of Mn is relatively small. The effect of W on the formation temperature and the amount of Laves phase is significant, and the critical temperature of Laves reduces with lowering W content. Without adding austenite forming elements Co, Ni or increasing in the Mn content, ferrite phase can be avoided after normalizing-and-tempering heat treatment through appropriately adjusting the content of W, C, and other elements.