The present invention is related to a replenishment method and device in an assembly line. A usage storage area for storing a first object set and a spare storage area for storing a second object set are defined in the replenishment apparatus. The replenishment apparatus comprises a fixing carrier in the usage storage area and a moving carrier in the spare storage area. The replenishment method comprises driving the moving carrier to move the second object set in the spare usage area to the fixing carrier in the usage storage area when a remaining number of objects in the first object set carried in the fixing carrier is zero and the first object set in the fixing carrier is removed; and moving the moving carrier to leave the fixing carrier and recover to the spare usage area.