Centrifugal fan is one of the important parts of the combine harvester, and study on its internal flow field distribution is of great importance for the structural design of the fan. Single duct centrifugal fan is commonly used in air-and-screen cleaning device of the existing full-feed rice combine. And it has the problems of unstable internal flow field, uneven outlet velocity and fast wind speed attenuation. In order to solve those problems, the channel model of the fan was established by using Solidworks software. Meshing it with ICEM software and simulating it in Fluent software, the internal flow field in centrifugal fan was simulated. The results showed that the wind speed at upper outlet and external rim of impeller was larger, and the pressure of impeller runner increased along radial direction. The wind speed of lower outlet Ⅰand Ⅱ and Ⅲ increased successively, and the decline rate decreased, which was useful for the air flow covering the whole sifter. Transverse air flow was nearly layered distribution on the upper outlet. Transverse air flow of middle outlet among three lower outlets was the largest, and that of other two outlets were symmetric. Maximum value of wind speed, air quantity and pressure of upper outlet and lower outlet Ⅱ and Ⅲ increased with fan speed increasing. Maximum value of each outlet’s air quantity and wind speed decreased with inlet diameter decreasing and the change of lower outletⅠwas more obvious. Variation of upper and lower separating board angle had a great change on wind speed and air quantity of lower outlet. The research of this paper provides basis for designing double outlet and multi-duct centrifugal fan.