The invention relates to an overlaying layer stripping ultrasound detection imaging system of a major-diameter thick-wall pressure vessel, comprising a mechanical scanning device, an upper computer of a control system, a lower computer of the control system, an ultrasound data processing imaging system and the like; wherein the mechanical scanning device further comprises a magnet-wheel adsorption assembly, a mechanical beam assembly, a vertical movement assembly, an ultrasound probe fixture assembly and the like, the mechanical scanning device is fixedly arranged on the wall of the mechanical scanning device in an adsorption way by the magnet-wheel adsorption assembly, the upper computer of the control system and the lower computer of the control system drive a DC servo motor to drive the ultrasound probe fixture assembly to accomplish coarse scanning or refined scanning, the magnet-wheel adsorption assembly has regulating action and can adapt to the detection of thick-wall pressure vessels with different diameters, the imaging processing system can generate intuitive stripping area plane image; the invention uses modular design, which is convenient for field application. The system is stable in operation, and the combination of automatic control and manual control can satisfy control required by different operations, thus being suitable for overlaying layer stripping detection of thick-wall pressure vessels in petrochemical engineering or nuclear power industry.