Abstract: Analyzed in the article are the main aspects of the on-line rapid water-cooling system for the hot stretch-reduced hi-frequency welded pipe, including the process characteristics, the equipment configuration and the operating principle, etc.Also studied are the mechanical properties and the metallographic structure of the Φ139.7 mm×7.72 mm TG26 HF welded pipe as has been on-line rapidly water-cooled.Results from relevant inspection and testing of the pipe show that owing to the on-line cooling and cooling control process, the microstructure and the properties of the hot stretch-reduced HF welded pipe have been enhanced, and accordingly, operation costs of the follow-up off-line heat treatment of the pipe is tremendously reduced, and the production efficiency enhanced.Onthe-site application result shows that the said cooling system exhibits high operation effectiveness.