A rotary drilling rig comprises a revolution platform and a back support leg device arranged on the revolution platform. The back support leg is formed by a back support leg and a supporting seat for installation of the back support leg, wherein the supporting seat is fixed on the revolution platform; the supporting seat is formed by a supporting base plate and an upper side baffle plate over the supporting base plate; the back support leg is formed by an anti-falling plate and an installation plate arranged under the anti-falling plate; the anti-falling plate is arranged over the upper side baffle plate; the installation plate is arranged under the supporting base plate; a first fixing hole is formed in the supporting base plate; the installation plate is provided with a second fixing hole matched with the first fixing hole; and fasteners are disposed in the first and second fixing holes to fixedly connected the supporting base plate with the installation plate. The back support leg device of the rotary drilling rig possesses great load bearing capability and great whole rig stability and can improve drilling precision; bolt stress of the balance weight and fixed balance weight can be avoided; and safety of the rotary drilling rig can be improved.