The underwater flood discharge noise is a high-intensity noise generated by hydraulic engineering, which has potential impacts on fish ecology. In order to explore the impact of underwater flood discharge noise on fish stress responses and physiological injury. In this study, juvenile grass carp (Ctenopharyngodon idellus) were exposed to the underwater flood discharge noise for two durations (15 and 30 min) and four sound pressure levels (110, 120, 130 and 140 dB) indoors experiment and 1.0-2.2 km downstream of Gezhouba Dam to compare the stress response and physiological injury of C. idellus juveniles. The results indicate that: (1) In the indoor broadcasting experiment, under the condition of 15-minute exposure to underwater flood discharge noise, the opercular beat rate and pectoral wing rate of C. idellus juveniles initially increased and then decreased as the sound pressure level (SPL) increased, there was a significant difference in cortisol concentration between the experimental group and the control group, but there was no significant difference between the 130 dB and 140 dB experimental groups. When the exposure duration was extended to 30 minutes, the opercular beat rate positively correlated with SPL, while the pectoral wing rate initially increased and then decreased with the increase in SPL, there was a significant difference in cortisol concentration between the experimental group and the control group. The response-weighted index (RWI) positively correlated with SPL, but there was no significant difference in RWI values under different exposure durations at the same SPL. (2) In the field experimental group, the opercular beat rate, pectoral wing rate and cortisol concentration of juvenile grass carp were significantly higher than those in the control group. RWI showed a significant positive correlation with both exposure duration and sound pressure level. The RWI of C. idellus juveniles in the field experiment was significantly higher than that observed in indoor experiments. When the exposure duration was 15 minutes, the proportion of severe damage reached 19.92%, and this proportion increased to 28.76% with a 30-minute exposure duration. With the increase in exposure duration, the proportion of severe injuries rises, while the proportion of moderate injuries declines, and the increase in the proportion of minor injuries is relatively small. Studies have shown that compared with the control group, underwater flood discharge noise increases the stress response of C. idellus juveniles and high-intensity underwater flood discharge noise can cause physiological injury to fish. This research contributes to the assessment of the impact of hydropower engineering construction on fish ecology, providing scientific basis and management suggestions for aquatic organism protection.