The utility model discloses a detect device of steel construction depression bar unstability, including the outer support ring of magnetic screen, interior support ring, scalable magnetism memory detection pen and power running gear, interior support ring installs the magnetic screen outside in the support ring, and power running gear installs on the outer wall of the backstop ring of the rear end of magnetic screen outer support, and in the support ring, the steel construction depression bar passes interior support ring including the installation of scalable magnetism memory detection pen, and power running gear can walk on the steel construction depression bar, and scalable magnetism is remembered detection pen and is used for detecting the steel construction depression bar. The utility model discloses a scalable magnetism memory detection pen has arranged two three -dimensional magnetism memory sensors and distance sensor altogether to hoop free motion through the device can realize normal direction and complete, the accurate collection of tangential magnetic signal, and can realize the detection to all kinds cross -sections member, guarantees the accurate judgements to all kinds of component stability that await measuring.