Conducted numerical simulation on the air flow distributions inside variable cross-section duct and the outlet of the duct by the method of computational fluid dynamics. Investigated the pressure, velocity distribution, outlet supply air volume and wind direction in different spatial section of duct. Analyzed the uniform features of variable cross-section duct air supply as well as the internal relations between uniform air supply and variable cross-section duct. With Reynolds numbers Re as variable, simulated the air distribution at the working conditions of Re =10 000，50 000，100 000 and 500 000. The simulated results indicates that in a certain range, as Re is larger, the static pressure distribution in duct is more uniform and the uniformity of the air supply is better; while Re is over 100 000, the uniformity variation of air supply is not obvious. It confirms that the balance function of variable cross-section duct to the duct static pressure distribution is main reason for uniform air supply.