Abstract: Abstract: A single-color steel open shed has been widely used for the main structure of the building in the production of fattening lambs. However, the roof cannot resist the external solar radiation in summer, leading to the increase of temperature in the shed and the occurrence of heat stress of lambs. The objective of this study was to investigate the heat insulation performance of a single-color steel roof that was covered with the reflective film, with emphasis on the physiological and growth performance of fattening lambs. Two sheds with similar building structures were selected, where one was reformed by pasting a reflective film on a single-color steel roof (film shed), and another without reform as a control (control shed). An infrared thermal imager was used to investigate the inner surface temperature of the roof, where the radiant heat of inner surface of the roof was calculated in each stage. Temperature and humidity in both sheds were continuously and dynamically measured, together with the growth performance and physiological indexes of fattening lambs. The trial lasted 67 days from June to August, including the three experimental stages (days 1-7 in the early-stage, days 8-37 in the mid-stage, and days 38-67 in the late-stage). The results showed that: 1) The inner surface temperature of the control shed reached the highest value at 11:30, while the film covered shed reached the peak value at 14:00, indicating a delay by 2.5 hours. The radiant heat of inner surface of the roof also showed the same pattern during the whole period. The inner surface temperature of the film shed was 1.87-14.20 ℃ lower than that of the control (average 7.31 ℃), indicating the most significant decrease (P<0.05) from 9:00-16:30 every day. In addition, the radiant heat of inner surface of the roof in the film shed was 8.85% lower than that of the control, indicating the largest 18.65% decrease in the late-stage at 11:30. 2) In three stages, the ambient temperature in the film shed decreased by 0.55 ℃ on average, compared with the control, especially in the early stage from 9:00 to 13:00, reaching 1.55-1.95 ℃ (average 1.78 ℃) (P<0.05). There was no significant temperature difference between the two sheds in the rest stages (P>0.05). Also, there was a significant linear positive correlation between the ambient temperature and the inner surface temperature of the roof, as well as the radiant heat of inner surface of the roof (P < 0.01). 3) The average daily gain of lambs in the film shed increased by 10.97 % (P < 0.05). There was a significant difference in the amount of water consumption between the fattening lambs in mid- and late-stage of the test (P < 0.05), which was 15.58 % and 12.66 % higher than that in the control. The respiratory rate of sheep in the film shed decreased by 5.31%, 7.85 times/min lower than the control (P < 0.05) in the middle stage. In summary, the single-color steel roof pasted by the reflective film can be expected to effectively improve the thermal insulation performance of the roof, and then to improve physiological and growth performance in the fattening lambs to relieve heat stress.