The box type hot air dryer with controlled temperature and humidity was designed. The drying machine mainly consists of internal recycle fan, heating system, humidifying equipment, weighing unit, and automatic control system. The experiment result showed that under 60 ℃ and 20% relative humidity, internal temperature and relative humidity were uniform and drying time was consistent in different positions with no significant differences. The material temperature changing curves seemed to be consistent under the same enthalpy value drying condition. Additonally, step-down relative humdity dyring method improve drying efficient a lot.