A magnetic adsorption type surface mounting device comprises an adsorption assembly and a surface mounting assembly. The surface mounting assembly is arranged on a supporting rack in the adsorption assembly, and the surface mounting assembly can slide on the supporting rack in the adsorption assembly, and is fixed on the supporting rack in the adsorption assembly through a locking nut. The adsorption assembly adsorbs the device on the internal surface of a longitudinal weld in a cylindrical workpiece made of a ferromagnetic material, and the surface mounting device makes a camera bag filled with films closely mounted on the internal surface of the detected segment of the longitudinal weld. The surface mounting operation of the internal surface of the longitudinal weld in the cylindrical workpiece made of the ferromagnetic material is completed outside the workpiece, so an operator is prevented from entering the workpiece during surface amounting, and the labor intensity is reduced. Thedevice reduces the lower limit value of the internal diameter of the cylindrical workpiece of a useable source in an outer single-wall viewing scheme, and avoids the reduction of the detection sensitivity of the workpiece, caused by a used source in outer double-wall viewing. The device has the advantages of simplicity in operation, rapidness and accuracy in close mounting of the camera bag filled with the films on the internal surface of the longitudinal weld in the cylindrical workpiece, close lamination of the films to the weld, improvement of the detection efficiency, and high detection sensitivity.