The present disclosure provides a bearing assembly, a rotor assembly and a draught fan. The bearing assembly has a shaft sleeve and a shaft extending through the shaft sleeve. An inner wall of the shaft sleeve is provided with two grooves in a circumferential direction. The shaft is provided with two channels corresponding to the two grooves respectively. The channels and the grooves form two raceways. Multiple rolling bodies are disposed between the shaft and the shaft sleeve and movable in the raceways. The shaft sleeve has an outer diameter of 13 mm, and a portion of the shaft extending within the shaft sleeve has a diameter of 5 mm.