Abstract: For the conventional magnetic flux leakage detector could not meet the requirements for inspection of long-distance oil and gas multi-diameter pipelines, a mechanical structure of magnet probe of multi-diameter detector was developed. The new structure is in the form of floating blocks, and the overall magnetic circuit structure is optimized by arranging probes in a staggered way in the 40-48 in (1 in = 2.54 cm) multi-diameter detector. As shown in the results of the magnetizing test, the mechanical test and the overall traction test, the mechanical structure has sufficient magnetization, and the probe is tightly close to the inner wall of the multi-diameter pipeline without any interference, capable of realizing the full coverage of the probe and adapting to the requirements of through capability of the multi-diameter pipeline. Generally, the reasonable mechanical structure of the floating magnet probe is the precondition for the successful corrosion inspection of oil and gas multi-diameter pipelines, and the research results will provide reference for promoting the development of the magnetic flux leakage inspection series technologies for the multi-diameter pipelines in China.