The heat transfer deterioration that occurs in the inner rain zone weakens the thermal performance of natural draft wet cooling towers (NDWCTs). Existing NDWCT retrofit methods including the air deflectors and the cross wall have limited effects on this deterioration. In this paper, we propose a new retrofit method in which air ducts are installed in the rain zone and air deflectors are installed around the air inlet to improve the total tower thermal performance. To clarify the effect and mechanism of our retrofit method, a hot test for a NDWCT model is performed under various crosswind velocities, and a 3D numerical model for a NDWCT with air deflectors and air ducts is established and validated. Using the proposed method, the thermal performance of a NDWCT is substantially improved with less crosswind sensitivity. It is found that the flow diversion efficiency of the air deflectors weakens the adverse impact of the ambient crosswind on air inflow of the tower, and the additional ambient air introduced through the air ducts enhances the heat transfer in the central rain zone. Compared with the single effect of the air deflectors or the cross wall, the combined effect of the air ducts and air deflectors is more efficient in improving the thermal performance of NDWCTs. (C) 2018 Elsevier Ltd. All rights reserved.