A 3D printer (1, 1a) includes a material rack (20, 20a). The material rack (20, 20a) includes a positioning plate (21) and a plurality of partition members (22). The partition members (22) are fixed to a door plate (12, 12a) and disposed in an inner area (120). The inner area (120) includes a plurality of material receiving compartments (121). A material tube (2) includes a pair of wings (3). The partition members (22) include positioning recesses (220) correspondingly. The material tube (2) is positioned by disposing the wings (3) on the positioning recesses (220) in a spanning manner. The material tube (2) is driven by a printing module (40, 40a) to move out of the material receiving compartment (121) and be positioned on the printing module (40, 40a).