The invention discloses a device for restraining a laser-induced plasma in high power laser deep penetration welding. The device comprises a cooling copper block, a floating gas curtain and a positioning part. The cooling copper block comprises two axisymmetric parts, an arc-shaped surface is arranged towards a weld joint side, and the cooling copper block is cooled relying on circulating water; the floating gas curtain is arranged on the bottom of the cooling copper block and uniformly sprays inert gas with a certain flow; the positioning part is used for fixing a restraining structure on a laser welding operating head and keeping the relative positions of the restraining structure and the laser welding operating head fixed. Meanwhile, the invention discloses a method for restraining the laser-induced plasma in the high power laser deep penetration welding. According to the device and the method, expansion of the laser-induced plasma in the three-dimensional direction is restrained in a high power laser welding process, the amplitude of fluctuation of the laser-induced plasma is reduced, thus a stable welding process and good formation of weld joints are obtained, and the device can be widely applied to the high power laser deep penetration welding field.