Abstract: Illumination is an important factor in aquaculture. Exploring the changes in the plasma physiology and biochemistry of Euthynnus affinis can provide references for the regulation of its optimal environmental factors. We designed four sampling time to investigate the light intensities on sunny days 2.7 lx (6:00), 1 116 lx (12:00), 913 lx (18:00) and 1 lx (24:00) and those on cloudy days 1.9 lx (6: 00), 698 lx (12:00), 192 lx (18:00) and 1.4 lx (24:00), so as to study the changes in the plasma cortisol, total cholesterol, triglyceride, malondialdehyde content, myeloperoxidase activity, lysozyme activity and immunoglobulin M concentration. The results show that there was no significant difference in the cortisol, malondialdehyde concentration and lysozyme activity among the four groups on sunny days, but they all reached the maximum at 18:00 on cloudy days. Besides, the concentraions of cortisol and malondialdehyde were significantly higher than those at 18:00 on sunny days (P<0.05), while the lysozyme activity was significantly lower (P<0.05). The total cholesterol content reached the maximum at 12:00 on sunny day and 18:00 on cloudy day, with no significant difference on sunny day and cloudy day at 12:00. However, it was significantly higher at 18:00 on cloudy days than on sunny days. The triglyceride content and myeloperoxidase activity reached the maximum on both sunny and cloudy days at 6:00, without significant difference. The immunoglobulin M concentration reached the maximum at 24:00 on both sunny and cloudy days, and it was significantly lower on sunny days than on cloudy days at 24:00 (P<0.05). The results show that the E. affinis is more suitable for surviving on sunny days, but it can adapt to the changes of light intensity in different weather by its active physiological adjustment, thereby reducing the damage of environmental factors to its physiological and immune functions.