A new detection device based on the principle of magnetic flux leakage was researched. The detection principle,the device’s structure and the parameter determination was discussed in detail. 32 sensors distributed uniformly along the circumferential direction were designed to ensure complete radial detection. By using the screw pair, its adjustable diameter can range from 400 to 600 mm which improved the working adaptability. The introduction of the flexible structures made elbow detection and passing-through available.This design not only can be used in blast furnace gas pipes in metallurgical enterprise, but can also adapt to the non-destructive examination for metal pipes in other fields.