The cooling water system of the Cont inuous Caster No. 1 in Baoshan Iron and Steel Co., Ltd. is an open circulat ing water system. Since 1989 the temperature of the inlet w ater in summer had of ten exceeded the mould alarm w ater temperature. After 1997 this problem had become more serious. In 1999 the t reatment program of the w ater system w as opt imized. The bott leneck problem w as successfully solved ever since. By now,cycles of concentration of this system is 1. 4 and the total cost is less than w hat it w as before. Most causes for this problem are summarized and the w ater quality and the mould condit ion are analyzed, before and after the new prog ram has been carried out