For the sake of achieving uniform drying and separation of alfalfa leaves and stems in one process, a quad-pass rotary drum dryer-separator was designed, which mainly comprised the pre-drying, quick-drying, tempering and separating zones. The air flow in the dryer-separator was simulated by using CFD method with applying the standard k—ε model, and the effect of different opening areas of the adjusting gate on the velocity in the separating chamber was analyzed emphatically. It was found that the desired velocity for leaf-stem separation in the separating chamber was gained, when the opening area of the adjusting gate was (150×150)~(200×150)mm2. 