The invention discloses a one-time forming tool for cross arm holding iron. The one-time forming tool comprises a base and a mold fixed to the base. The mold comprises a profiling mold body and a forming pressing plate. The forming pressing plate comprises a connecting lug, a pressing plate body and a pressing rod connecting piece. The connecting lug is connected to one end of the pressing plate body, and the pressing rod connecting piece is connected to the other end of the pressing plate body. The top face of the profiling mold body is a circular arc face. One side of the circular arc face is an inclined plane, and the other side of the circular arc face is a groove. A pin shaft hole is formed in the profiling mold body. The central line of the pin shaft hole coincides with the central line of the circular arc face. The connecting lug of the forming pressing plate is hinged to the profiling mold body through a pin shaft. The pin shaft is fixed in the pin shaft hole of the profiling mold body. The two ends of the pin shaft extend out of the pin shaft hole to be connected with the connecting lug of the forming pressing plate. According to the one-time forming tool, the triangular holding iron is formed through one-time pressing and forming of flat iron, the lever principle is applied ingeniously in the forming process, time and labor are saved, the machining efficiency is high, the specifications of the machined holding iron are unified, and consistency is good.