A fan rotor protection structure includes a fan wheel and an annular member. The fan wheel has a hub and a plurality of blades. The hub has a top portion and a side wall portion, which together define a receiving space in the hub. The annular member has one side aligned with and connected to an end surface of the side wall portion opposite to the top portion, and defines an opening communicable with the receiving space in the hub. With the annular member, foreign matters are stopped from entering into and accumulating in the fan rotor, enabling largely increased fan service life and more stable flow field around the fan.