The invention discloses a multi-blade centrifugal ventilator with cylindrical denoising structure; air conditioner noise is a main noise source in daily life, whereas centrifugal ventilator noise is a main part to the air conditioner noise. The multi-blade centrifugal ventilator with cylindrical denoising structure comprises a volute, a mounting disc, a blade wheel and a denoising cylindrical cage; the blade wheel is disposed between air inlets in two sides of the volute, and the blade wheel and the volute form a revolute pair; the denoising cylindrical cage is composed of a connection ring, an adjusting ring and denoising cylinders, which are integrally formed. One denoising cylinder is disposed at the inner side of each blade to reduce the flow of gas impacting directly on the blade so that the noise of the multi-blade centrifugal fan is reduced effectively; in addition, the denoising cylinders arranged at the inner side of the blade wheel can break a tail flow zone at an exit of the blade wheel, so that gas flow loss is reduced and flow field stability of the ventilator is improved.