A shaft brake mechanism of wind power generator, including a first brake assembly and a second brake assembly independent from each other. The first brake assembly serves to provide braking effect for the shaft of the wind power generator against rotation. The second brake assembly serves to naturally restrain the rotational speed of the shaft from exceeding a nominal upper limit of rotational speed. Accordingly, the wind power generator can still safely operate in a situation that the wind speed exceeds a nominal upper limit of wind speed. Therefore, the wind speed range for the operation of the wind power generator is widened to increase the total power generation capacity thereof.