Abstract: The boiler tube of DZS1-0.7-Y steam boiler ruptured in Xinmin pump station in 1993. The study and analysis showed that the imperfect water treatment, unmatched accessary of boiler and frequent switchovers between water and steam were the rupture cause. The modification has been made against the rupture: adopting combined water treatment system to improve the water quality; replacing the fins and reducing the frequency of switchovers.