A box forming device includes a placing module, a bearing seat, tongue folding modules and a second side wall folding module, wherein the placing module can enable a paper plate to be inserted into areceiving space of the bearing seat, and is folded to form a first side wall of a box through a first side wall pusher of the bearing seat. The tongue folding modules are used for folding the tongue of the first side wall. One end of the second side wall folding module is rotatably connected with the bearing seat for folding to form a second side wall of the box body, and the second side wall is pushed by the inner wall portion of the second side wall folding module. The inner wall portion is wrapped to the inner side of the tongue. Thereby, a plate is folded into a box by the cooperation of the placing module, the bearing seat, the two tongue folding modules and the two second side wall folding modules.