The invention relates to a detector for detecting a steel pipe via a rare earth permanent magnetism material, and belongs to the field of steel pipe wall thickness measuring and quality defect detection. The detector detects the thickness and pipe wall condition of a detected object according to magnetic field and pressure change between and the detected pipe, early warning is raised against thinning, crack and leakage points in the pipe wall, a mobile device carrier is composed of two independent parts, control is convenient, and the mobile device carrier is arranged in a sealed way to adaptto different detection environments. The detector has the advantages that the detector can realize detection of relatively high precision in a severe pipe environment, early warning is raised againstthinning, crack and leakage points in the pipe wall, the designed mobile device carrier is higher in sealing performance, behaviors are convenient to control the structure is simple and practical, andthe detector is highly practical and economical and can be applied to production practices.