The invention discloses a pneumatic mechanical arm device for steel bar bundling. The pneumatic mechanical arm device comprises two pneumatic devices which are oppositely arranged up and down and a forming ring. Each pneumatic device comprises an inner hollow air cylinder body and an air cylinder base, wherein a pneumatic piston is arranged in the air cylinder body, the piston is provided with a piston rod, a trapezoid sliding block is fixed to the upper end of the piston rod in a nested manner, a pair of inclined sliding grooves are formed in the two end positions of the side face of the sliding block correspondingly, and a T-shaped sliding groove and an upper opening are formed in the air cylinder base. Two sliding block driven pieces which oppositely move are arranged in the T-shaped sliding grooves, and each sliding block driven piece is embedded in the inclined sliding groove of the corresponding trapezoid sliding block. A wire screwing mechanical pawl is fixed to the sliding block driven piece located on the upper air cylinder base, and a wire grabbing mechanical pawl is fixed to the sliding block driven piece located on the lower air cylinder base. The method that the pneumatic mechanical pawls clamp an insulation bundling wire for rotation tight screwing is adopted in the pneumatic mechanical arm device, the steel bar bundling efficiency is improved, meanwhile, the bundling wire is not prone to damage, and the requirement of automatic steel bar bundling in the building industry is met.