The invention discloses an envelope forming device, which can adapt to packing materials with different specifications and different sizes. The envelope forming device comprises a rack, an upper fixing frame and a lower frame parallel to each other are arranged on the rack, the relative positions of the upper fixing frame and the lower fixing frame can be adjusted, a pair of upper mounting seats which can slide along the axial direction to be positioned are arranged on the upper fixing frame, the upper mounting seats on the left side and the right side are respectively provided with a left upper forming block and a right upper forming block, the lower fixing frame is provided with a pair of lower mounting seats which can slide along the axial direction to be positioned, and the lower mounting seats on the left side and the right side are respectively provided with a left lower forming framework and a right lower forming framework. The forming device can be constructed through the leftupper forming block, the right upper forming block, the left lower forming framework and the right lower forming framework with adjustable positions, thereby realizing the free adjustment of the forming size of the forming device. Therefore, one forming device can adapt to forming requirements for different specifications and different sizes, thereby greatly reducing the manufacturing cost. Moreover, due to the simple and rapid installation and adjustment, the invention reduces the time of installation and maintenance and improves the efficiency.