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Objective: In order to explore the effect of step-down relative humidity drying strategy on the drying characteristics of agricultural products. Methods: Yam slices were selected as the object. The effects of constant humidity drying （relative humidity 15%, 25%, 35%, 45%） and step-down relative humidity drying （relative humidity 45% in the first stage for different times of 10, 20, 30, 60 min and relative humidity 20% in the second stage） on the drying characteristics of yam slices were studied at 60 ℃; The multi-field coupling model was constructed for the simulation of heat and mass transfer; The rehydration ratio and microstructure of the dried product were also measured. Results: ① During constant humidity drying, the drying rate decreased with the increase of relative humidity; However, the drying time of step-down relative humidity drying （45% relative humidity maintained for 20 min and then decreased to 15%） was 8.3% shorter than that of constant humidity drying （relative humidity 15%）. ② The drying rate of yam slices generally increased first and then decreased, The fitting results of drying model showed that it was accurate to describe the changing process of moisture and mass transfer. ③ The rehydration ratio increased first and then decreased with the increase of relative humidity; Under special drying condition of step-down relative humidity drying strategy （relative humidity 45% kept for 20 min and then decreased to 15%）, the yam slices showed honeycomb porous structure, which was good for the mass transfer. Under this condition, the maximum rehydration ratio was 6.85 ± 0.05; Under the drying condition of constant humidity strategy （relative humidity is 20%）, the microstructure of yam sllices begin to shrink and collapse, which led to a lower rehydration rate. Conclusion: During the process of hot air drying, the multi-field coupling model can significantly shorten the drying time, effectively improve the microstructure of materials and improve the rehydration rate of products; The multi-field coupling model can precisely simulate the heat and mass transfer process in yam slices during hot air drying; The research results can provide theoretical basis and technical support for the application and optimization of step-down relative humidity drying strategy.