Abstract: Abstract: Box type hot air dryer has been widely used for agriculture products because of its simple structure and convenient operation. How to shorten drying time, reduce energy consumption and improve products' quality becomes one of the important directions of research. As to the material which easily crusts in the surface, it is expected to use high relative humidity pretreatment and then reduce the relative humidity to enhance drying rate. In this circumstance, the temperature and moisture gradient between sample and drying air become consistent and high efficient drying process can be achieved. However, it is difficult to confirm the magnitude of relative humidity and its keeping time in each step during drying process. It is expected that material's temperature and relative humidity of hot air are the main 2 factors that have a significant influence on the drying process. High temperature of hot air drying leads to high temperature of material, so that drying time can be reduced. Material's internal moisture movement towards surface can be accelerated because of material temperature increment. Besides, the vapor partial pressure difference between material's surface and drying medium can be increased by reducing drying relative humidity, which can improve drying rate. In addition, material's temperature and relative humidity of drying hot air are the 2 aspects which reflect the mass transfer and heat transfer of the drying procedure. High relative humidity can increase material's temperature but decrease drying rate. Conversely, low relative humidity may increase drying rate but hinder increment of material's temperature. The drying procedure can be divided to initial preheating period, medium-term temperature increment period and late drying period based on material's temperature curve change. So according to material's temperature, drying period can be specified and then relative humidity can be controlled so that drying process can be optimized. In this paper, the control of relative humidity was designed for internal recycling hot air drying technology. In initial preheating period, relative humidity was set to a high value. And the material's temperature was raised rapidly. When the slowly increasing trend of material's temperature was detected by the automatic control program, the drying process transferred to medium-term drying procedure. In this period, material's temperature was kept at a specific value with the moisture removing fan open. When the material's temperature began to increase, the moisture removing fan was closed and the material was heated. When material's temperature approached drying hot air temperature or material's temperature couldn't rise rapidly, drying process transferred to late drying period. Carrot was adopted to test the performance of the automatic control program of relative humidity. The experiment result showed that in the way of self adaptive control of relative humidity, the maximum deviation of relative humidity was 1% in the preheating period, and the drying temperature of material kept under dehumidifying increased gradually. The maximum error between set temperature and achieved temperature was 0.8℃. In the last drying period, the judge on the drying end time by the moisture content in 15 min was 9 min later than the judge by weighing method. Drying time was reduced by 19.7% compared to the drying condition of 50% relative humidity in the initial period and 20% in the late period as well as the drying condition of 50% relative humidity pretreatment and removing moisture continuously afterwards. The present work comes up with a new way of controlling relative humidity in internal recycling hot air drying technology, which is useful for enhancing drying rate.