In this paper we present a solution that addresses the reflectance loss due to oxidation. The solution is a capping layer (CL) that acts as an effective oxidation barrier when the multilayer (ML) is exposed to the extreme ultraviolet (EUV) light in the presence of water vapor, so that the optical element of multilayer can be used as long as possible. The theoretical reflectivity of ML Mo/Si EUV mirrors is calculated at 13.9nm under the different CL materials. The thicknesses of the CL and standard ML are optimized by the simplex method, meanwhile the reflectivity of ML can be further enhanced when the layer by layer theory is used. Finally the electric-field intensity distribution in the top layers of the ML is analyzed when the CL is added.