Abstract: Relative humidity (RH) can be gradually reduced to improve the drying efficiency and quality of materials in hot air drying. The high RH in the early drying stage can rapidly heat up for better diffusion of internal moisture in the material. The internal water can migrate and then spread to the surface of the material. The high RH can reduce the water vapor partial pressure difference between the drying medium and the material surface, in order to inhibit the evaporation rate of water on the surface, and then prevent the surface from drying and crusting. The moisture removal includes two procedures during drying: the internal moisture diffusion to the surface, and the evaporation of surface moisture. Once the surface moisture evaporates too fast, and the internal moisture cannot be supplied to the surface in time, the surface of the material is the first to shrink, which is the direct cause of crust hardening. After the material surface crusts harden, the drying time is prolonged to reduce the rehydration rate and the drying qualities. In the early drying stage, the high RH can improve the internal moisture diffusion rate. Additionally, the surface evaporation rate is reduced. A surface with enough moisture can alleviate the crusting of the material surface for a better drying and rehydration rate. However, it is still lacking in the quantitative comparison of internal moisture diffusion and surface water evaporation. It is also unclear about the process and mechanism of crust formation. Therefore, it is very necessary for the quantitative description of the correlation between the internal moisture diffusion and surface water evaporation on the crusting, in order to optimize the relative humidity for better drying efficiency and quality. In this study, the internal moisture diffusion quality (D), moisture evaporation quality (E), material surface moisture accumulation (Q), material microstructure, and rehydration ratio were investigated under three RH control strategies, including the constant RH (20%, 30%, 40%, and 50%), the RH 50% with different time (10, 30, 60, and 90 min), and the auto RH control strategy using material temperature. The results showed that the D increased gradually with the drying time, and then remained stable under constant RH drying. The E increased gradually with the drying time and then decreased. Specifically, the higher RH was, the faster the material heating rate was, and the larger D was. The lower RH was, the greater E was. When the RH was 20%, 30%, and 40%, the time of Q=0 was 1.11, 1.36, and 1.70 h, respectively. After this time, the material surface presented outstanding crusting. Besides, the higher RH was, the later the time of crusting was. When the RH was 50%, there was no Q<0, indicating no outstanding crusting. When Q>0, the drying rate was consistent with the changing trend of Q value. When Q<0, the corresponding drying rate decreased. When the RH 50% was maintained for 30 min and then reduced to 20%, the time when Q equals 0 was 1.39 h. Compared with the drying condition of RH 20%, the material temperature and internal moisture diffusion rate increased to delay the timing of crust formation. After decreasing the RH, E increased, and the drying time was shortened by 18.5%. Under automatically controlled RH, Q increased rapidly within 0-0.25 h, corresponding to a rapid increase in the drying rate. Q gradually decreased within 0.25-0.50 h. Within 0.78-2.00 h, there were three zeros in Q value, indicating the fluctuation at Q=0. This RH control mode can be expected to serve as the moisture migrating from the inside to the surface evaporate in time without accumulation on the surface. The temperature of the material showed a stepwise upward trend, and then postponed the appearance time of crusting, to retain more water migration channels. In this optimal drying condition, the shortest drying time was 6.1 h, and the highest rehydration ratio was (4.39±0.07) g/g, as well as the lowest shrinkage ratio was 28.55%±1.71%. The automatically controlled RH was the optimal RH controlling drying. This finding can provide a theoretical basis and technical support for the internal migration and surface evaporation of water, particularly for the cause of surface crusting and the optimization of the RH control during drying.