Disclosed is a structure for reducing noise of a ventilating fan. The structure includes an orifice plate fixed on a scroll casing and is shaped to be an annular and semi-surrounding hollow structure in order to match with the shape of an air inlet of the scroll casing, and to cover only the air inlet of the scroll casing. The noise generated by an air blower of the ventilating fan can be directed along a bell mouth of the scroll casing of the ventilating fan after being emitted from the air inlet of the scroll casing, and then can be smoothly sucked by the orifice plate. The sucked-in noise can repeatedly collide with the orifice plate and be diffused, thereby the energy being gradually weakened and the sound pressure thereof being decreased.