Abstract: Abstract: Open sun drying has been widely applied in the developing country. It is a popular, effective and economic method for drying and preservation of agricultural products and food, but it has some drawbacks such as degradation of materials quality and larger drying period in ambient environment. In order to solve the problem of uncontrollability and randomness in the process of open sun drying, heat and moisture transfer model of open sun drying materials was studied with theoretical knowledge of heat and mass transfer in this study, and heat and moisture transfer mechanisms of open sun drying materials were analyzed. The heat exchange process of dry materials in the open sun drying process mainly included the absorption of solar radiation heat, the convective heat exchange with the surrounding environment, the heat transfer of long wave radiation and latent heat by water evaporation. Thermal resistance and thermal energy diagram of drying materials was built. Heat balance equation of materials was also built based on energy balance principle, and model parameters were determined by comprehensively considering solar radiation intensity, ambient air temperature and humidity, and ambient wind velocity; the dynamic temperature change of material surface and the water transfer rate of material were predicted, the program was solved by using MATLAB software, and the initial values required for the solution of the model were derived from the experiment data. To verify the accuracy of prediction model, sweet potato was selected as drying material in the open sun drying test. The tested samples of sweet potato were sliced uniformly in terms of size and thickness, the initial mass of sweet potato was 240 g, and the thermal performances and drying characteristics of sweet potato in the open sun drying were investigated in the Qianfo mountain campus of Shandong University. The test period was from 8:30 to 17:00 on May 7, 2017. The test parameters included solar radiation intensity, outdoor air temperature and humidity, surface temperature and mass change of sweet potato during the drying process. Fourteen mathematical models were tested to specify the suitable model for describing the drying behavior of the tested samples, and it was found that the Page model was convenient to describe the drying characteristics of sweet potato in open sun drying. During the test period, the solar radiation intensity ranged from 398.91 to 979.9 W/m2, the air temperature ranged from 27.2 to 41.2 ℃, and the sweet potato mass decreased from 240 to 57 g. The determination coefficient were respectively 0.96, 0.89 between the simulated and experimental values in terms of drying materials temperature and mass, and root mean square error were respectively 0.97 ℃, 28.35 g, and the change trend of simulated and experimental value was consistent, but there were certain differences between them, and especially the difference of dry material mass was large. The main reason for this phenomenon was the model calculation parameters and experimental data had some irrationality. It is concluded that the model can predict accurately the material dynamic change of surface temperature and drying mass rate in the process of open sun drying of materials, and this model can be used in the process control of open sun drying.