Abstract: In the hot air drying process, step-down relative humidity (RH) means that RH is gradually reduced to improve the drying efficiency and quality. The step-down RH hot air drying method has been successfully applied to the drying processing of yam slices, Dahongpao pepper, papaya slices and other materials. In the drying process, how to regulate the relative humidity in stages to improve the efficiency and quality is the key point of the research on the stage of dehumidification. At present, there are three main stage dehumidification control methods: two-stage dehumidification, multi-stage dehumidification drying and RH control method based on material temperature. RH has a significant effect on both internal moisture migration and surface moisture evaporation. The changes of internal moisture migration and surface moisture will be reflected in the changes of RH. When RH shows an upward trend, it indicates that the increase of surface moisture evaporation makes RH rise. When RH shows a downward trend, it indicates that the internal water moisture increases and the surface water evaporation is not enough to make RH rise. RH control policy based on RH changing was as follows. In the early drying stage, the evaporation of the material itself made the RH rise. When the RH became stable, it was transferred to the middle drying stage. In the middle drying period, when RH showed a downward trend, the dehumidification was closed and the RH was increased to reduce the surface water evaporation (E) and increase the internal water migration (D). When RH was rising, the dehumidification was opened and the RH was decreased to increase E. If the moisture evaporation on the surface was not enough to increase RH, or the temperature of the material approached the drying medium temperature, the drying was transferred to the later stage. In the late drying period, dehumidification was opened to reduce RH to increase E value. When the change rate of RH was less than a certain range, the drying end point was reached. Under drying temperature 60 ^oC and air velocity 3.0 m/s, the drying test results of carrot showed that 0-8 min was the early drying stage, and the RH change rate was less than 0.5% at 8 min. Afterwards, the carrot entered the middle drying stage. During the drying period from 8 to 131 min, the time of RH decreasing trend was gradually shortened. The time of RH rising trend was gradually extended. Within 137-142 min, RH no longer showed a rising trend and entered the late drying period. After 142 min, it was the late drying stage. At 370 min, the RH change rate was less than 1%/min, and the drying process ended. The RH control mode increased D value and decreases E value, so that D and E values remained basically equal within 0.2～2.3 h. Meanwhile the material temperature presents a step-rising trend in corresponding. Within 0～0.2 h, it indicated that the accumulation of water (Q) raised rapidly. The material was wrapped by a layer of water film and no obvious crust phenomenon occurred. Within 0.2～2.3 h, Q value fluctuated up and down at zero point and drying rate decreased gradually, producing 3 zero points in total. The water migrating to the surface immediately evaporated on the surface without accumulation, delaying the time of crust occurrence and removing a large amount of water. After 2.3 h, Q is gradually less than 0, and the surface of the material produced obvious crust phenomenon and gradually thickened. The drying time was 5.6 h, the rehydration ratio and shrinkage rate were (4.41±0.02) g/g and (27.32±1.51)%, respectively. Compared with the constant 20%RH, the drying time was shortened by 24.6%, and more water migration channels were retained. The RH control method realized automatic control of RH, improved drying efficiency and quality, and provided theoretical basis and technical support for RH control during hot air drying of fruits and vegetables.