The roof ventilated desiccant bed is a passive building energy-saving device that directly utilizes solar energy for building insulation and indoor dehumidification. Its operation involves significant thermal-moisture coupling effects, with complex heat and moisture transfer mechanisms and computational processes. This study analyzes the heat transfer process of the roof-integrated passive regenerative drying bed under direct solar radiation. A simplified heat transfer model was established based on the lumped parameter method and energy conservation principles, with model parameter identification accomplished through COMSOL simulation data and the differential evolution algorithm. Further comparative analysis of computational results between this simplified heat transfer model and the numerical model under multi-cycle dynamic boundary conditions indicates: The simplified model demonstrates excellent agreement with numerical simulations, where the mean absolute error (MAE) of temperatures at characteristic nodes remains below 1.2 °C; the MAE of heat flux density at the inner surface is 1.71 W/m2, with a mean absolute percentage error (MAPE) of 2.9% during the 10:00–16:00 period. Furthermore, its computational efficiency far surpasses that of the numerical model: the numerical model required approximately 48 minutes to complete the aforementioned case study, while the simplified model completed the same computation in merely 2.9 seconds on the same computer.