The utility model discloses an air seal for a bearing of a water turbine generator. The air seal comprises a shaft sleeve (1) which is sheathed on a large shaft of the generator, a bearing housing (2), a positive pressure pipe (3), an air seal ring (4) which is sheathed on the shaft sleeve (1) and is fixedly connected with the outer wall of the bearing housing (2), a negative pressure pipe (5) and an oil mist pumping device (7), wherein the positive pressure pipe (3) is connected with the air seal ring (4); and the one end of the bearing housing (2) far away from the air seal ring (4) is connected with the oil mist pumping device (7) through the negative pressure pipe (5). The air seal for the bearing of the water turbine generator has the beneficial effects that since a complete pressure drop is formed through positive pressure - atmospheric pressure - negative pressure, oil mist in the bearing housing is prevented from overflowing along the shaft and is enabled to be pumped out only through the oil mist pumping device, and oil mist pollution is prevented from being caused in a generator pit.