The invention discloses a hydraulic sealing method and a sealing ring for an injection height-adjusting bridge steel support. A trapezoidal notch is formed at the bottom of a piston of the bridge steel support; the sealing ring is sleeved on the trapezoidal notch; and a support basin and a piston are in clearance fit. The sealing ring is arranged at a position of the bottom face of the support basin before the support piston is arranged at the position of the bottom face of the support basin, so the sealing ring extrudes to form a primary seal. When a height-adjusting filler is filled through a filling opening, in the process that the upper structure of the support is lifted by high-pressure hydraulic pressure, the end face and the radial inner side of the sealing ring are subjected to the high-pressure hydraulic pressure and the internal force of the sealing ring, and the sealing ring is extruded to be deformed and is kept static relative to the support piston to form a high-pressure static seal. On the radial outer side of the sealing ring, the filler compresses the long lip side of the sealing ring to retreat to the lip opening cavity of the sealing ring through upward and outward hydraulic pressure, so a space occupied by the long lip side becomes shorter, the long lip side extrudes and is tightly adhered to a basin structure and the sealing ring slides upwards relatively to form a high-pressure dynamic seal on the radial outer side of the sealing ring.