The invention discloses a device preventing oil mist of a generator bearing from overflowing. A sealing cover is correspondingly arranged between a groove cover and a rotary shaft to form a sealing cavity. N elastic sealing pieces are arranged between the sealing cover and the rotary shaft, and (n-1) sealing cavities arranged in sequence are formed in the outer surface of the rotary shaft. N is larger than or equal to 4, and at least sealing cavities are formed. A blow-off opening is formed in the upper sealing cavity, an air suction opening is formed in the middle sealing cavity, and an oil slinger arranged on the outer surface of the rotary shaft is located in the lower sealing cavity. An oil return hole is formed in the lower portion of the sealing cover to be communicated with a cavity inside the oil groove cover. According to the device, oil mist generated when a generator set operates can be sealed, is limited in an oil groove and is difficult to disperse outwards. The oil mist cannot overflow and cannot enter air to pollute components around a power generation assembly. A high-pressure fan blows to offset external negative pressure, the oil slinger blocks rising lubricating oil, the oil mist is absorbed and separated through an oil mist absorption device, and the aims of preventing negative pressure and shed oil and purifying the oil mist can be achieved.