The invention discloses a protecting frame for construction of a prefabricated external wall board with a bay window and belongs to the technical field of building construction. The protecting frame is characterized in that each support of the protecting frame is a right-angle triangular frame made of square steel, each right-angle triangular frame is provided with bolt holes capable of vertically penetrating a right-angle plane, and the bottom side of each square steel on the horizontal side of each triangular frame and through holes allowing bolts to penetrate in the vertical direction integrally form steel right-angle triangular protecting frame supports; three bolts penetrate three bolt holes in each triangular protecting frame support to symmetrically fasten the steel right-angle triangular protecting frame supports on two vertical frame walls of the bay window, a scaffold board on a right-angle dual-purpose board is fastened on the steel right-angle triangular protecting frame supports by bolts, and the scaffold board and a protecting board are vertically mounted to form the right-angle dual-purpose board. The protecting frame has the advantages that a protecting frame system is wide in suitable range and free of limitation of wall size and window size, the protecting frame supports are quite convenient to mount and demount, and only few workers are needed for mounting and demounting, so that manpower cost and construction period are reduced, safety coefficient is high, and the protecting frame is practical.