The invention provides a welding method for a copper back plate and a target, comprising the following steps of: placing the copper back plate and the target in a sheath, and disposing a ventilation tube in the sheath; placing the sheath in a furnace, and vacuumizing the sheath via the ventilation tube; after the interior of the sheath achieves a certain vacuum degree, increasing the temperature of the furnace to a certain temperature, continuously insulating the furnace for a period of time, and still vacuumizing the sheath during the insulating period of the furnace; sealing the ventilation tube in the sheath; and performing hot isostatic pressing welding on the sheath provided with the copper back plate and the target in the interior, so as to weld the copper back plate with the target together. During the process of vacuumizing the sheath, the sheath is continuously kept in a high-temperature environment, so that liquid water or other liquids in the interior of the sheath form steam capable of being extracted away, so as to form a real vacuum environment in the interior of the sheath, thereby obtaining a target assembly with high welding strength and yield.