Abstract: Abstract: To improve the summer environmental conditions for open-sided beef barns in Southern China, a wet curtain cooling fan-fiber duct system was designed to locally cool the barns based on air jet distribution. The wet curtain cooling fan was used as a cooling source with air volume at 13 000 m3/h and 150 Pa of total pressure. A fiber duct was used as the end of the duct of the air distribution system to deliver cold air evenly to the beef activity area for local cooling. The duct was 0.8 m in diameter and designed with four rows of holes at the bottom with diameters of 20.0 mm and 22.0 mm, respectively; the spacing between two adjacent holes along the axial direction of the duct was 58.0 mm and 60.0 mm, respectively. The air dispensing mode of the system was jetting. The duct was hung above the stalls, and bottom of it was 1.75 m from the floor, and 0.96 m from the manger. Considering the local environmental conditions and farm management, the operation time of the cooling system was from 08:00 to 20:00. The study was conducted with Simmental beef in an open-sided barn with feed and water available ad libitum. Data of wind speed along the duct in the axial direction, the cooling effect of the wet curtain cooling fan, the environmental parameters of the barns, and the physiology and body weight gain of beef were collected. Air speed was 8.39 m/s at the head of the fiber duct and 8.21 m/s at the end of the duct. The variation coefficient of wind speed among 10 cross profiles was 0.0153, which showed that the air duct could be considered uniform in air supply, with rational design and layout of holes. The outlet temperature of the wet curtain cooling fan increased with ambient temperature and humidity rising. During the measurement period, at beef standing height (1.3 m from floor) and at reclining back height (0.7 m from floor), compared with the control barn, average wind speed increased by 0.69 and 0.47 m/s, respectively (P<0.01); the average temperature of the treatment barn decreased by 2.0 and 1.8℃, respectively; and, the average relative humidity increased by 10 percent at two heights. However, humidity was still within the appropriate range of 55-85 percent. Compared with the control barn, the average mass concentration of ammonia and carbon dioxide in the treatment barn decreased by 0.17 and 81 mg/m3, respectively; and the average HLI decreased by 2.84 (P<0.01). Compared with beef in the control barn, the respiratory rate of the experimental beef was decreased by 22 beats / min, rectal temperature was decreased by 0.35℃, and skin temperature was decreased by 1.04℃ (P<0.01). Throughout the experimental period, the beef in the treatment barn never experienced any sickness, and daily weight gain was 0.92 kg/d; while the morbidity rate of beef in control barn was up to 47 percent, and daily weight gain was 0.54 kg/d (P<0.01). Operation of this system for two months could earn 13 564.96 Yuan of profit. The results showed that the application of a wet curtain cooling fan-fiber duct in an open-sided beef barn was effective in relieving heat stress of beef. The application of this system in Southern China, which is hot and humid, is technically and economically feasible.