A self-compressing high-temperature downhole packer comprises a setting mechanism mounted in a pipe column, a sealing mechanism mounted on the setting mechanism and a fixing mechanism mounted on the sealing mechanism. The setting mechanism comprises a central pipe, a body head connected with the central pipe, an inner cylinder and an outer cylinder, wherein the inner cylinder and the outer cylinder are connected with the body head. A piston is arranged in a piston cavity formed by the inner cylinder and the outer cylinder, and high-temperature sealing rings are mounted in an inner sealing groove and an outer sealing groove of the piston correspondingly. The lower end of the outer cylinder is provided with a shoulder which limits the piston moving distance, and the outer cylinder is connected with a limiting ring. The lower end of the outer cylinder is provided with a conical end face which forms an annular space with the limiting ring. A locking sleeve is mounted in an annular space formed by the inner cylinder and the outer cylinder and makes contact with the lower end face of the piston, and the locking sleeve is pushed to move downwards through moving of the piston. The problemthat after setting of a thermal recovery well, the sealing mechanism loses efficacy under influence of temperature change is solved, and the technical problems that the fixing mechanism cannot be fixed and unsetting cannot be conducted after setting are further solved.