The invention discloses a high-temperature sealing device and method for a downhole packer. The device comprises supporting bodies, a sealing element, separation parts, control step-by-step seat sealing mechanisms and seat sealing and pressing mechanisms, wherein the separation parts are symmetrically mounted at the two axial ends of the sealing element; one control step-by-step seat sealing mechanism is mounted on each of the separation parts; the separation parts are fixed on the packer through the control step-by-step seat sealing mechanisms; each of the seat sealing and pressing mechanisms is arranged at the outer axial side end of the packer. Each of the supporting bodies is between the corresponding separation part and the corresponding seat sealing and pressing mechanism and is extruded between the inner side surface of the seat sealing and pressing mechanism and the outer side surface of the separation part by the seat sealing and pressing mechanism. According to the high-temperature sealing device, the packer achieves a good sealing effect.