The invention provides a device for pressurizing fluid in a casing pipe of a pumping unit well. A pressurizing auxiliary module is included, and in the module, one end of a piston rod of a piston stretches out of a shell, and the other end of the piston rod is connected with a piston head arranged in the shell; a cavity of the shell is divided by the piston head into a first cavity body and a second cavity body provided with an elastic part; a liquid inlet is located in the portion, with the first cavity body, of the shell, and a liquid outlet connected with a casing pipe head of a Christmas tree is located in the portion, with the second cavity body, of the shell; a liquid outflow check valve is arranged on the liquid outlet, and a liquid inflow check valve is located on the piston head;the two ends of a pipeline communicate with the liquid inlet and a storage tank correspondingly, and the height where the storage tank is located is larger than that where the pressurizing auxiliary module is located; one end of a polished rod is connected with a pumping unit, and the other end of the polished rod is connected with a pumping rod; and a first cover plate is located on the potion, between the pumping unit and the Christmas tress, of the polished rod, is driven by the polished rod to move up and down, and makes contact with one end, stretching out of the shell, of the piston rodin the downward moving process, and therefore cost is reduced, and the safety of operation is improved.