The invention provides an automatic molding device of a carton box. The device comprises a transmission device for moving the carton box along the transmission direction, a feeding device for gradually feeding the folded carton box, an initial forming device for raising one side surface of the folded carton box to form an opened carton box, a folding dispensing device for folding and molding a front cover, a rear cover, an upper cover and a rear cover of the opened carton box and dispensing to adhere to form a molded carton box, a rack and a control device; the feeding device, the initial molding device and the folding dispensing device are sequentially arranged on the rack in a sequence along the transmission direction; the control device is respectively connected with the feeding device, the initial molding device, the folding dispensing device and the transmission device through signals. With the adoption of the device, the carton box can be automatically molded, thus the labor intensity of a worker can be reduced, the labor cost can be saved, and the production efficiency can be raised.