The invention discloses an automatic tensioning mechanism for an accessory head of a square ram with a small cross section. By the aid of an idle cylinder, a pull claw tensioning mechanism and a wedge device, the accessory head of a machine tool is grabbed and loosened on the square ram with the small cross section. The automatic tensioning mechanism comprises a mounting seat, a flange cover, the idle cylinder, a safety pin, a large pull claw, a spindle, a spindle bearing and the square ram. The automatic tensioning mechanism is flexible and compact in structural design, safe and reliable in operation and simple in manufacture and assembly, is mainly applied to the machine tool industry of a vertical turning-milling center and the like, meets the requirement of the accessory head in the square ram with the small cross section on automatic tensioning, reserves the advantages of the prior art, has huge market advantages and makes full use of the potential of the machine tool.