The invention discloses a side protection device of external wallboard construction of a prefabricated building, and belongs to the technical field of building protection. The side protection device of the external wallboard construction of the prefabricated building comprises an external wallboard, and further comprises a clamping mechanism, a protection mechanism and a guarantee mechanism. The clamping mechanism is arranged on the top of the external wallboard and used for clamping the external wallboard. The protection mechanism is arranged at one end of the clamping mechanism and used forprotecting workers. The guarantee mechanism is arranged at the bottom end of the clamping mechanism, connected with the external wallboard in a threaded mode and used for providing the supporting force for the protection mechanism when the clamping mechanism is damaged. The external wallboard is vertically arranged. The side protection device can be integrally moved after the construction is accomplished, the repeated disassembly and assembly operation is avoided, and the time is saved. When the side protection device is installed, only the protection mechanism needs to be inserted into a through hole, the installation can be accomplished, the operation is convenient and fast, and disassembly and assembly are convenient. According to the side protection device, due to the fact that the guarantee mechanism is arranged, when the clamping mechanism is damaged, the guarantee mechanism continues to provide the supporting force, the side protection device is prevented from falling down, andthe reliability is improved.