Abstract: The weld in condenser cooling tube of power plant was perforated after chemical cleaning. In order to find out the cause and mechanism of perforation, the analysis was implemented by scale composition analysis, SEM microstructure, water analysis etc. The results show the main composition of scale is calcium carbonate and contains some corrosive elements such as sulfur and phosphorus. The hard scale blocks the flow of corrosion, which caused under deposit corrosion. Plenty of carbon dioxide was generated after chemical cleaning. The weld of cooling tube is relatively weak and finally the perforation occurred due to various factors. Additionally some reasonable suggestions are provided at the end of article.