The present disclosure provides an internally self-circulating fluidized bed gasifier and an air distributor therein for generating a stepped constrained wind. The air distributor includes a gas-material mixture through hole and a plurality of vent holes. Each of the vent holes is designed to have a winding path. Due to the winding paths of the vent holes and an arrangement of converging outlets of the vent holes to the gas-material mixture through hole, which is in communication with a furnace chamber, the present disclosure enables a gas entering the furnace chamber to form a stepped constrained wind and effectively prevents solid-phase materials in the furnace chamber from leaking into a gas mixture chamber. The internally self-circulating fluidized bed gasifier can achieve self-circulation combustion gasification for multiple times and a well-controlled gasification temperature, resulting in a high coal gasification efficiency without an ash leakage.