The development of industrial applications of extreme ultraviolet lithography (EUVL) severely based on surface oxidation and carbon contamination on using extreme ultraviolet (EUV) reflective mirrors. The capping layer coating on the Mo/Si multilayer is a promising method to improve the stability and service life of the reflective mirrors. The capping layer is prepared by direct current reactive magnetron sputtering. The "hysteresis loop”of oxygen flow and sputtering voltage is established, the required oxygen flow for different oxides is accurately grasped. Choosing the RuO2 and TiO2 as the capping layer materials; the different film characteristics in each state is analyzed. For TiO2 films, crystal phase, surface roughness, cross section uniformity and chemical composition are evaluated. 97.2% TiO2 content (mass fraction) of Ti-oxide is obtained; reflectivity of Mo/Si multilayer with 2 nm TiO2 as capping layer is controlled in 5%; surface roughness less than 100 pm, dense and uniform amorphous TiO2 films is fabricated to make this capping layer achieve the basic requirements of EUV.