The vertical separation principle of alfalfa in alfalfa rotary drum dryer was studied, and the wind field of separation exit of the three pass rotary drum dryer was optimized by using CFD based on the range of separation velocity when the gas temperature was about 90℃. An optimum structural dimension between the bottoms of the middle and outer drums of a triple pass rotary drum dryer, modified to achieve drying and leaf-stem separation of alfalfa in one process, was recommended based on the CFD simulation results of a series of the contours of velocity and wind field of velocity vectors at different dimensional conditions. 