Abstract: Plant factories have become an important development direction in the field of modern agricultural engineering due to their advantages of year-round continuous production and precise environmental regulation. However, as the vertical and horizontal dimensions of planting region expand, maintaining a stable and uniform internal environment becomes increasingly challenging. Continuous heat release from artificial light sources exacerbates the thermal load, making the internal microclimate regulation in large-scale plant factories particularly complex. These conditions often result in disordered airflow, uneven temperature distribution, and localized heat accumulation, all of which significantly impact the growth of leafy vegetables. Conventional single circulation ventilation system is generally inadequate for meeting the environmental control demands of large-scale plant factories, as their limited airflow penetration often fail to ensure uniform temperature regulation across the planting region. The study investigated the temperature uniformity control performance of a double circulation upward return air system featuring both sides horizontal air supply and vertical roof return air of the planting region. The temperature distribution law and flow field characteristics in the large-scale plant factory are analyzed through the combination of actual measurement and CFD simulation methods. The actual measurement results demonstrated that the temperature difference in the 1 m/s horizontal air supply region reached up to 3.8 ℃, while the corresponding difference in the 10 m/s horizontal air supply region was 2.2 ℃. Horizontally, the temperature distribution exhibited a characteristic of being higher in the center and lower on both sides, with the high temperature zone on the side of the 1 m/s horizontal air supply region. Vertically, significant stratification was observed, with heat accumulating in the upper layers. The maximum temperature difference across the entire planting region reached 3.8 ℃, indicating uneven temperature field. The CFD simulation results further revealed that the low-velocity airflow (1 m/s) from the left and the high-velocity airflow (10 m/s) from the right converged forming a vortex region where airflow velocity dropped below 0.3 m/s. This low-velocity zone restricted heat dissipation and led to the formation of a localized high-temperature zone. Due to the difference in air inlet velocity between the left and right sides, as well as the obstruction caused by facilities such as the nutrient tank on the right side, the main body of the airflow convergence zone is located in the low velocity air inlet regulation zone, occupying 16.9% of the area of the low velocity air inlet regulation zone. In order to balance the large difference in air inlet velocity between the left and right air inlet walls, which leads to different adjustment capabilities on both sides, additional CFD simulations were carried out by increasing the left side inlet velocity to 1.5 m/s and 2.0 m/s. As the velocity on the left side increased, the cold air penetration depth improved, and the vortex region gradually shifted rightward. When the left side inlet air velocity is 2 m/s, the area of the vortex zone decreases to only 5.2%, representing a 42% reduction in the vortex range compared to when the left-side inlet air velocity is 1 m/s. The thermal mass is reduced, and the temperature field across the entire space tends to become uniformly distributed. This study provides a reference for the design of ventilation systems in large-scale plant factories.