[Objectives] To investigate the effects of high temperature stress on Hippocampus abdominalis seedlings, 20-day-old seedlings were subjected to acute high temperature stress for 24 h, and the survival rate and antioxidant enzyme activity were measured at different temperatures and times. [Methods] The seedlings were transferred directly from 18 ℃ to 7 temperature gradients of 22, 24, 25, 26, 27, 28 and 30 ℃, and the number of deaths and behavioral status were recorded every 2 h, and a time-temperature-survival model was constructed. Probit regression analysis was used to determine the temperature at which the mortality rate reached 50% in 24 h, which was considered the high initial lethal temperature (24 h-UILT50). According to the results, the seedlings were directly transferred from 18 ℃ to 21 ℃, 24 ℃ and 27 ℃ for 24 h. Samples were collected at stress 0, 3, 6, 12 and 24 h, respectively, and the antioxidant oxidase activities at different temperatures and different time points were measured using biochemical kits. SPSS 26 software was used for data processing, and one-way ANOVA and Duncan test were used for significance analysis. [Results] The results showed that in the high initial lethal temperature experiment, the H. abdominalis seedlings did not die between 18﹣25 ℃, they began to die slightly at 26 ℃, and died completely at 30 ℃, and the survival rate decreased with the increase of water temperature. Probit regression analysis showed that the high initial lethal temperature was 27.53 ℃ (Fig. 1, Table 1). The activities of superoxide dismutase (SOD) and catalase (CAT) showed the same trend in the 24 h high temperature stress experiment. At 21 ℃ and 24 ℃, the activities of SOD and CAT first decreased and then increased. The activities of SOD and CAT increased at 27 ℃, and were significantly higher than those of control group at 24 h (P < 0.05) (Figs. 2, 3). The content of malondialdehyde (MDA) increased at the initial stage of stress, and did not change significantly at 21 ℃. After 6 h of stress, the content of MDA began to decrease in the 24 ℃ treatment group, and after a period of stress at 27 ℃, the content was significantly higher than that in the control group (Fig. 4). In all temperature groups, total antioxidant capacity (T-AOC) showed an increasing-decreasing-increasing trend with stress time, and all decreased to the lowest level at 12 h (Fig. 5). [Conclusion] The experiment showed that acute high temperature stress significantly changed the activity of antioxidant enzymes in H. abdominalis seedlings, which were activated to eliminate reactive oxygen species (ROS) generated by high temperature stress, and the content of MDA in H. abdominalis seedlings was significantly increased, indicating that the seedlings were still under oxidative stress. There were significant differences in the trend of antioxidant enzyme activity between the 27 ℃ treatment group and the other temperature treatment groups, indicating that H. abdominalis seedlings could not adapt to the high temperature at 27 ℃.