[Objective] Loess is loose and porous, and hence contains much air. Soil water vapor movement in the soil is a complex process, in which water and air, two phases of fluids, drives and affects each other in vertical movement in the soil. So it is essential to get to know how dry air flow affects soil water and heat transfer to agricultural production in the Loess Plateau.[Method] To explore effect of soil dry air on soil moisture and heat transfer in an apple orchard of loess on a hill, the STEMMUS model (Simultaneous Transfer of Energy, Mass and Momentum in Unsaturated Soil)was adopted to simulate soil hydrothermal dynamic process. Numerical simulation is a feasible and economical method to reproduce the process.[Result] Results show:(1) the model is a useful tool to well simulate the dynamic process of soil water and heat in the soil. By optimizing the soil hydraulic parameters, in simulation of soil moisture druing the calibration and verification periods, the model varied in the range of 4.7%-30.0% in normalized root mean square error and in the range of 0.83-0.96 in consistency index, while in simulation of soil temperature, it did in the range of 0.1%-9.9% in normalized root mean square and in the range of 0.76-0.99 in consistency index; (2) the simulation using the two-phase coupling STEMMUS model with the dry air mechanism taken into account was more approximate to the measured value than the single-phase STEMMUS model without taking the dry air mechanism into account. After 1 day of rainfall, the increment of soil moisture content in the soil profile of the coupled model was obviously lower than that in the soil profile of the single-phase model. The coupled model was 0.07% in normalized root mean square error, and from 0.93 to 0.97 in consistency index, while the single phase model varied in the range of 0.06%-0.2% normalized root mean square error, and in the range of 0.95-0.96 in consistency index. Considering the existence of pressure infiltration in this area, the coupled model can better reflect the actual soil water movement.[Conclusion] Compared with traditional models, the STEMMUS model can better reflect the actual soil water and heat dynamics in loess by taking soil dry air mechanism into account, which is favorable to designing optimal management strategies for orchards on the Loess Plateau.