This study presents a comprehensive computational and experimental investigation of a cabinet-type solar dryer specifically designed for the dehydration of Plantago major leaves, a widely used medicinal plant. A detailed three-dimensional transient model was developed using ANSYS Fluent to simulate airflow behavior, solar radiation heat transfer, and temperature distribution within the drying chamber under natural convection conditions, considering solar irradiance levels ranging from 600 to 850 W/m² and ambient temperatures of 28–35°C. The initial CFD results revealed temperature gradients of up to 18.2°C between trays and the formation of stagnant airflow zones, leading to uneven drying and significant moisture variation of up to 11.8% across trays. To address these limitations, the design was iteratively optimized by repositioning air inlets/outlets, integrating deflector plates, and adjusting tray spacing from 8 cm to 5 cm. The improved configuration resulted in a 25.4% enhancement in thermal uniformity (measured as standard deviation of tray temperature), a 22.1% reduction in total drying time (from 6.8 hours to 5.3 hours), and a 17.3% increase in overall energy efficiency. Experimental validation was conducted using 1.5 kg batches of freshly harvested Plantago major leaves, with moisture content monitored using gravimetric methods. The improved dryer reduced final moisture content variation between trays to less than 2.5%, ensuring more uniform drying and improved retention of bioactive compounds such as aucubin and flavonoids. These results confirm the predictive power of CFD modeling for solar dryer optimization and validate the practical benefits of the improved design. The integration of simulation and field data highlights the viability of deploying sustainable, energy-efficient, and low-cost drying solutions for medicinal plant processing, particularly in off-grid and rural communities.