Abstract: Heat stress has been one of the most important factors in the dairy industry against global warming in recent years. The milk performance of dairy cows can be confined to long-term heat stress. The purpose of this experiment was to investigate the effects of spoiler fans cooling on milk performance and immune function in dairy cows. Two cowsheds were used with the same building structure and system. The spoiler fan were installed, while the control cowshed was uninstalled any fans. The lactation performance (including milk yield, 4% fat-corrected milk, milk protein rate, milk fat rate and lactose rate) and lymphocyte apoptosis rate were detected in different experimental periods; Meanwhile, blood biochemical parameters and their mRNA expression were also investigated, including four kinds of cytokines (Interleukin-1 (IL-1), Interleukin-2 (IL-2), Interleukin-6 (IL-12), and Interleukin-12 (IL-12)), six kinds of apoptotic factors (BAK, Mcl-1, Bax, Bcl-2, Bcl-xl, and P53), and three kinds of immunoglobulins (Immunoglobulin A (Ig A), Immunoglobulin G (IgG), Immunoglobulin M (IgM)). Besides, Spearman correlation analysis was carried out between milk performance or immune parameters and equivalent temperature index for cattle (ETIC). Approximately 200 Holstein lactating dairy cows were kept in each cowshed. The whole experiment period was 42 days from June to August, including four stages: initial (0 d), early (1-14 d), middle (15-28 d) and late stage (29-42 d). The results showed that: 1) The installed fans improved the cowshed environment, where the ETIC was reduced by 3.52 ℃ during the entire period, compared with the control cowshed. Cows in the control shed were in the moderate heat stress, while cows in the fan-installed shed were in the mild heat stress. Thus, the fans significantly alleviated the heat stress in cows. 2) Installation of fans improved the production performance of dairy cows exposure to heat stress. The milk yield, 4% fat-corrected milk, and milk fat increased in the fan-installed shed during the middle and late periods of experiments, compared with the control shed (P<0.05). Particularly, the 4% fat-corrected milk increased by 39.1% during the middle period and by 35.2% during the late period, respectively. 3) Installed fans improved the immune function of dairy cows under heat stress. Blood lymphocyte counts during the middle and late periods were greater in the fan-installed shed than those in the control group (P<0.05), and their early apoptosis rates of lymphocytes decreased (P<0.05). Moreover, there was a reduction in the mRNA expression of pro-apoptotic factor Bax in lymphocytes and its serum level (P<0.05) during the late period. An increase was found in the mRNA levels of anti-apoptotic factor Bcl-xl in lymphocytes and its serum level was demonstrated during the middle and late periods. Besides, the installed fans increased the serum levels of IL-12 and IgG (P<0.05). 4) The correlation analysis showed that the milk protein rate and the lymphocyte count demonstrated a negative correlation with ETIC (P<0.05), and the BAK mRNA expression in lymphocyte and serum IL-6 level exhibited a positive correlation with ETIC (P<0.05). In conclusion, the spoiler fans in the shed can be expected to effectively relieve the heat stress in cows in summer, thus promoting the thermal environment and blood biochemical indicators in cows for better production performance and immune function.