The invention provides a method and device for improving the forming quality of laser additive manufacturing thin-wall parts. The device comprises a movable platform system, a laser metal deposition system, an auxiliary forming system, an active cooling system and a central control system. According to the forming method, when the laser metal deposition system conducts cladding forming on the thin-wall parts, the auxiliary forming system controls the thin-wall thicknesses and the forming directions in real time; when the multiple layers of the thin-wall parts are deposited, the problem of molten pool flowing caused by interlayer stacking can be solved, and the problem that the surface roughness in the thin-wall forming process is excessively large due to interlayer stacking is solved, andthe thickness-changeable thin wall, the flat thin wall and the curved thin wall are machined; and meanwhile, the active cooling system cools deposited layers in real time. The problems that the quality of the laser cladding formed surfaces is low; during continuous multiplayer depositing, the temperature is increased, therefore, thermal stress of the deposited layers is accumulated, consequently the deposited layers are cracked, and microstructures of the deposited layers are coarse; and the forming efficiency is low due to slow cooling are solved, and high-quality and efficient forming of thecomplex thin-wall parts is achieved.