In order to improve the phenomenon of increased flow separation caused by the influence of gas viscosity on the small multi-blade centrifugal fan, the influence of gas viscous torque on volute wall distribution was studied based on the traditional volute design theory, and the momentum correction was used to modify the design. In addition, in order to truly reflect the distribution of the flow field in the fan, the viscous stress was added to the diffusion term of standard k-εcalculation model to reduce the maximum calculation error to 3%. Comparing and analyzing the fan’s numerical simulation and experimental results before and after modification, the modified k-εmodel was used to calculate, and it was found that the vortex intensity in the modified fan was reduced, and the flow separation near the volute outlet at the volute outlet was improved. The experimental results showed that the static pressure at the outlet of the modified fan was increased by about 25Pa, and the maximum full-pressure efficiency was improved by about 10% compared with the prototype. At the same time, due to the expansion of the volute expansion, flow separation can be suppressed, the vortex intensity in the vicinity of the volute tongue and its influence area were reduced, and the noise of the multi-blade centrifugal fan was effectively reduced by 2.5dB.