The invention discloses a cold tightening method for bolts on split surfaces in a high-medium pressure cylinder of a half-speed unit. The cold tightening method comprises the following steps: 1, mounting the bolts one by one into connecting holes on the two split surfaces in the high-medium pressure cylinder in advance; 2, mounting a matched mechanical stretching nut on the bolt at the middle position of each split surface, simultaneously fastening the bolts at the middle positions of the two split surfaces through matching of a hydraulic fastening tool and the mechanical stretching nuts, andenabling the elongation of the bolts to reach a qualified standard to form first fastening bolts; and 3, mounting mechanical stretching nuts on bolts at the two ends of each split surface sequentiallyfrom the two sides of the first fastening bolts, and fastening the bolts on the two split surfaces at the same time through matching of the hydraulic fastening tool and the mechanical tension nuts toenable the elongation of the bolts to reach the qualified standard. The cold tightening method for the bolts on the split surfaces of the high-medium pressure cylinder of the half-speed unit has theadvantages of high accuracy, no damage to the bolts of the original cylinder, no loss of pretightening force, rapidness, and manpower and time saving and the like.