The invention relates to a pressure maintaining cylinder sealing structure. The sealing structure comprises a core barrel, an inner rod, drilling outer cylinders and flaps. The lower end of the innerrod is in sealing cooperation with the inner wall of the core barrel. A gas accommodation cavity, a liquid channel and a pressure releasing channel are arranged in the inner rod. A piston is arrangedin the gas accommodation cavity. The upper end of the liquid channel communicates with the gas accommodation cavity. The lower end the liquid channel is opened on the base face of the inner rod. A safety valve is installed at the entrance of the pressure releasing channel. The flap comprises a valve seat and a sealing valve clack. The valve seat is installed on the inner walls of the drilling outer cylinders. One end of the sealing valve clack is movably connected with the outer side wall of the upper end of the valve seat. When the core barrel is arranged in the valve seat, the sealing valveclack is started for 90 degrees. When the core barrel is upwards lifted to a certain height, the sealing valve clack is closed. According to the invention, by use of the flap, lower end sealing is achieved; during starting, the sealing valve clack is hidden between inner and outer cylinders, so space is saved and limitations on the diameter of a drilling core can be eliminated; by setting the pressure releasing channel and the safety valve on an upper end sealing part, pressure can be timely released, pressure in the cylinder can be quickly reduced and devices are prevented from being damaged.