Taking the multi-blade centrifugal fan in an air-conditioning system as the object, the three-dimensional modeling of the internal fluid of the fan is carried out. The steady-state and non-steady-state calculations are carried out in the CFD software, obtaining the internal flow field information, with calculation result imported into Noise estimation which was performed in the LMS Virtual.lab. Besides, the accuracy of the fan aerodynamic noise calculation was verified against the experimental results. Through the analysis of the mechanism of the aerodynamic noise of the fan, the main noise source of the aerodynamic noise of the fan is located at the impeller and is closely related to its internal flow field distribution and its structure. For the fan products that have been put into production, it is difficult to modify the main structure including the volute, the impeller, the blade size, etc., and only local structure can be redesigned to achieve the purpose of noise reduction. Therefore, the blade perforation design is adopted. By setting reasonable perforation parameters, the eddy current shedding around the blade can be reduced without changing its performance, so as to reduce the pressure pulsation of the blade surface and the aerodynamic noise of the fan.