The invention relates to an oil cylinder mobile locking mechanism of a jack hydraulic press, belonging to the technical field of pressure machinery. The jack hydraulic press comprises a stand, a workpiece frame, a jack body and a pitch-down device, wherein the stand comprises an upper cross beam, a lower cross beam, a first trailing arm and a second trailing arm; the workpiece frame is arranged on the first trailing arm and the second trailing arm and positioned between the upper cross beam and the lower cross beam; the jack body is positioned outside the first trailing arm or the second trailing arm; and the pitch-down device consists of an oil cylinder, a supporting plate and a pair of sliding guide mechanisms and adaptively arranged on the upper cross beam. The oil cylinder mobile locking mechanism comprises a screw rod, a sleeve nut and a hand wheel, wherein one end of the screw rod is connected with the pitch-down device, the other end of the screw rod is matched with the sleeve nut, the sleeve nut is rotatably arranged on the upper cross beam, and the hand wheel and the sleeve nut extend to the end part outside the upper cross beam for being fixed. The invention has the advantages that the processing quality of a workpiece to be processed is guaranteed, the safety is ensured, the working intensity of operators is lightened, and the pitch-down device can not collide with two ends of the upper cross beam.