Abstract: This study explored the effects of different photoperiods (long light 18L﹕6D, short light 8L﹕16D, continuous light 24L﹕0D, continuous darkness 0L﹕24D and control group 12L﹕12D) for 90 days on the growth, energy budget and relative expression of circadian clock genes on the juvenile koi (Cyprinus carpio). The results showed that the survival rate of each group had no significant change (P>0.05), and the feed coefficient of 24 L group and 18L﹕6D group was significantly lower than that of 12L﹕12D group, 8L﹕16D group and 24D group (P<0.05). The terminal body weight, feed coefficient and specific growth rate were not affected by light for the first 30 days of the experiment (P>0.05), but significantly changed for the last 30 days of the period. Prolonging light cycle, especially continuous light, significantly increased the terminal body weight and specific growth rate (P<0.05), while the feed coefficient decreased significantly (P<0.05). Compared with the continuous light and dark 240L﹕024D group, the ratio of koi fish feeding energy distribution in metabolism and growth increased significantly in other photoperiod treatment groups (P<0.05). The relative expression levels of Clock, Per2, Cry1 and Bmal1 biological clocks genes have no obvious chaenge. These findings indicate that prolonged illumination time, especially continuous illumination, is beneficial to the growth and development of juvenile fish in koi fish, combined with the experimental results of growth performance, energy budget and biological clock gene expression level under different photoperiods, while prolonged illumination or continuous illumination is beneficial to the growth and development of juvenile fish of koi.