The invention discloses a prefabricated box girder square steel pipe core die which comprises a core die body, multiple supporting frames and multiple collecting and releasing type strengthening supporting frameworks. The core die body is a hollow octagonal prism formed by multiple square steel pipes in a surrounding manner. The multiple supporting frames and the multiple collecting and releasingtype strengthening supporting frameworks are arranged in the hollow part of the core die body at intervals in the length direction of the core die body. The supporting frames and the collecting and releasing type strengthening supporting frameworks are arranged in a cross manner. Each supporting frame comprises an upper steel bar, a rightwards and upwards inclined steel bar, a right steel bar, a rightwards and downwards inclined steel bar, a lower steel bar, a leftwards and downwards inclined steel bar, a left steel bar and a leftwards and upwards inclined steel bar, all of which are sequentially connected end to end through a connecting mechanism. The core die is easy to assemble, mount and demount, the labor intensity is lowered, labor force is saved, the construction cost is reduced, the construction period is shortened, the supporting frames and the collecting and releasing type strengthening supporting frameworks are arranged in the core die body in the cross manner, the overall rigidity and strength of the core die are improved, and the phenomena of cracking and damage on the surface of the core die are greatly reduced.