The invention discloses a machining device for a construction steel bar and an application method thereof. The machining device comprises a straightening box. A first electric telescopic rod is fixedly connected to the top of an inner cavity of the straightening box. A supporting plate is fixedly connected to the bottom of the first electric telescopic rod. Upper conveying roller brackets are fixedly connected to the left and right ends of the bottom of the supporting bracket. Through action of the first electric telescopic rod, the supporting plate, the upper conveying roller brackets, upperconveying rollers, the upper conveying roller brackets, upper guide roller brackets, lower conveying roller brackets, lower conveying rollers, lower guide roller brackets, lower guide rollers, a second supporting seat, a second electric telescopic rod, a cutting rod, a first supporting seat, a reserved opening, a bearing, a rotating shaft, a second bevel gear, a rotating disc, threaded holes, threaded rods, anchor stocks, limiting plates, a ratchet wheel non-return mechanism, a first bevel gear, and a first motor, steel bar machining efficiency is effectively improved, great convenience is brought for application of users.