Abstract: Based on analyzing the main problems of ordinary mold powder used in casting of 09CuPRE series RE-treated steels at PZH Steel and the research upon effect of RE oxide(RE_xO_y) on the properties of mold powder,a prefused mold powder with low solidification temperatures and low crystallization temperature and strong ability of absorption and solution of RE_xO_y was developed. This mold powder is of suitable component,special fluxes and inferior oxidizability. Industrial experiment results show that the developed mold powder is satisfied with the demand of 09CuPRE series steels casting process, the ratio without slab surface finishing is >95%, no one breakout accident happened in the past 3 years.