In view of the current situation that a large number of abandoned support pile tubes of conduit rack platforms, torches and trestle bridges need to be removed, a set of internal hot-melt cutting device was designed and developed for cutting pile tubes with a diameter of 800 ~1800 mm, a wall thickness of ≤50 mm, a cutting speed of 200 ~700 mm/min and a cutting position below 5 m from the mud surface. The key components of the demolition device were designed and analysed for strength. The strength of the structural components of the demolition unit was verified and analysed under operational conditions by means of hot melt testing and comparison with finite element results. A prototype of the demolition device was designed and manufactured according to the scheme, and the functional suitability and operational reliability of the device were verified through quay tests and offshore simulations, thus providing guidance and reference for the design of offshore abandoned support pile tube demolition devices.