The invention relates to a pre-tightening device used for installing a transformer bushing. The pre-tightening device comprises a fixed mount, a hydraulic mechanism and a pressure application mechanism, wherein the fixed mount is fixed on a conservator; the hydraulic mechanism is borne on the fixed mount; the pressure application mechanism is used for applying pressure to a platen and is connected with the hydraulic mechanism; and the hydraulic mechanism drives the pressure application mechanism to move towards the conservator. The pre-tightening device can quickly press the platen of the transformer bushing, assembles a compression nut onto a conduction cylinder when required pre-tightening force is achieved, the platen is limited, and then, the pre-tightening device is removed to finish installing the transformer bushing. Therefore, one circle of bolts does not need to be screwed by a great quantity of manual work, workload is effectively lowered, and manual work man hour is shortened so as to reduce product cost.