In manufacturing industries such as automobile, home appliance and engineering machinery, etc, for the sheet metal uncoiling production line, the magnetic chuck used for steel plates stacking and conveying is the essential and important component, the usability and service life of which directly affect the working efficiency of the whole production line. It can hold up steel plates of δ0.5—2.5mm with an air gap or non magnetic block of δ10mm, and can convey normally with δ7mm. In the whole production line, electric permanent magnet and electric control permanent magnet are responsible for the final discharging with power off.