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The effect of relieving heat stress of cows was verified by three kinds of average spraying droplet diameter (SDD) (0.829mm,0.947mm and 1.127mm) through field test. In the test, the body surface temperature of dairy cow was collected by thermal imager;the respiratory rate and physiological index of rectal temperature were measured at the same time. Based on this, the theory of enthalpy difference of Merkel was used to calculate the heat transfer of spraying cooling. The results showed that during the process of spraying, for three kinds of SDD (0.829mm,0.947mm and 1.127mm), the average reductions of temperature were 0.7℃, 1.1℃ and 0.9℃ in the region of neck;in the region of abdominal, the average reductions of temperature were 0.7℃,1.4℃ and 1.5℃, respectively;and the average respiratory rates were decreased by 0.6, 4.2 and 2.1 times per minute, respectively. After spraying, for three kinds of SDD, the average reductions of temperature were 0.2℃, 0.4℃ and 0.6℃ in the region of neck;in the region of abdominal, the average reductions of temperature were 0.1℃, 0.5℃ and 0.6℃, respectively, and the average respiratory rates were decreased by -0.4, 1.4 and 1.2 times per minute, respectively. The rectal temperature was controlled within the stable range before and after spraying. The average rectal temperatures under SDD of 0.947mm and 1.127mm were lower than that under SDD of 0.829mm. Under three kinds of SDD, the heat exchanges of cows in unit time were 417.4W, 469.9W and 430.4W, respectively, which showed that heat exchange reached the maximum under SDD of 0.947mm. In summary, the SDD of 0.947mm was more suitable for the cooling of dairy cows in summer.