The invention discloses an oil mist sealing structure of a hydraulic generator and belongs to the field of power generation equipment. The oil mist sealing structure is composed of a zero-clearance sealing part and a magnetic liquid sealing part, achieves rotary combined seal and is applicable to bearing oil mist seal of the hydraulic generator from the low speed to the high speed. A sealing shell of a combined sealing device is formed by connecting four bearing sealing covers through bolts. Firstly, four sealing split bodies are installed on the lower portions of the bearing sealing covers, composite flexible materials are adopted in the circumferential direction to connect the four sealing split bodies, zero-clearance sealing is formed under the radial and axial thrust effect of a small spring, and bearing oil mist is well sealed; then, by means of the magnetic liquid seal on the upper portions of the bearing sealing covers, the zero-leakage sealing effect is achieved; by combining the two kinds of seal, air pressure of a sealing cavity formed by the upper seal and the lower seal can be effectively kept, no negative pressure is produced in the cavity, overflow caused by the pressure difference between the high internal pressure and low bearing sealing cover external pressure can be effectively prevented, and the service life of the oil mist seal is prolonged.