The photocycle can affect antler shedding and new antler regeneration in Père David's deer (Elaphurus davidianus) during winter solstice. The spillover effect of continuous light duration change in young stage on antler growth and reproduction during estrus has not been reported. In this study, 12 male milu deer born in Beijing Milu Park (April 5, 2018-April 20, 2018) were subjected to different light treatments in sequence during the experiment period from November 2021 to March 2023: Control group (natural light), Shortened light for 2h, Extended light for 3h, Extended light for 6h. The study aimed to monitor the continuous influence of light duration on milu antler shedding cycle and the regulation of new antler regeneration rate, analyze the relationship between light duration and the shed antler cross-section parameter factors, and continue to track the corresponding relationship between estrus ranking and fighting, mating and reproduction performance caused by the diagonal influence of light duration during antler growth. The results showed that continuous shortening of illumination for 2 h (6L∶18D) could advance the shedding date of milu by 8.5 d compared with that of control group (8L∶16D). Prolonged illumination for 3 h (10L∶14D) and 6 h (12L∶12D) delayed the shedding date by 10.5 d and 13.5 d, respectively, compared with the control group (8L∶16D). Compared with control group (8L∶16D), the Antler length increased by 5.47 cm (P < 0.01, n=6) and the weight increased by 18.64g (P < 0.01, n=6) when the illumination time was extended for 3 h (10L∶14D). Compared with the control group (8L∶16D), the Antler length increased by 10.43 cm (P < 0.01, n=6) and the weight increased by 44.31 g (P < 0.01, n=6) after 6 h (12L∶12D). Shortening the illumination for 2 h (6L∶18D) shortened the Antler length by 2.93 cm (P < 0.01, n=3) compared with the control group (8L∶16D), and reduced the weight by 9.81 g (P < 0.01, n=3) compared with the control group (8L∶16D). Prolonged illumination had significant difference on the weight of milu antler slice (P < 0.01, n=6). Prolonged illumination for 3 h (10L∶14D) and 6 h (12L∶12D) increased the weight of milu antler slice by (0.1200±0.0318) g/ slice and (0.3133±0.0618) g/ slice compared with control group (8L∶16D), respectively. The section weight of the shortened 2 h group (6L∶18D) was (0.1230±0.0561) g/ piece compared with that of the control group (8L∶16D). Prolongation of light in the young stage could significantly increase the mating opportunity and mating time of adult male milu, increase the duration of estrus and cumulative duration, increase the check of estrus of female deer, maintain territory and group, and improve the mating success rate. To sum up, there was a close correlation between old antler shedding and light duration in milu, and the change of sustained light duration at young age had spillover effects on the date and reproduction of adult milu antler. The mechanism still needs further study, and the results can provide theoretical support for future studies on the molecular regulatory mechanisms of different light on old antler shedding and the diversification of new antler regeneration in milu. It can guide the yield improvement, breeding, protection and sustainable development of deer antler.