The Xiaolangdi reservoir is one of the biggest reservoirs on the Yellow River which plays a very important role in regulating the environment about the river. Because the riverbed downstream is getting higher and higher with time elapses, special Sediment-Flushing-Process has to be taken annually since 2002. During this process, however, there are usually large-scale of "flowing fish" in Yellow River which has been ignored by the authority for some time already. Based on the related reports and our pre-investigation, Yellow River Carp Cyprinus carpio, which is one of the most important commercial fish in Yellow River, is one of the main species that suffers from the process. In order to investigate the internal causations of the "flowing fish" phenomena, we collected 6 samples for the control group ((872.9±59.97) g, Mean±SD) before the process and 6 samples for the experimental group ((765.7±93.37) g) during the process. The sampling location was on the shore 3km downstream to the Dam. All the 12 fish was killed immediately after collection and dissected on the spot, some physical index such as the length and the weight were measured, and the alimentary canal was fixed in the alcohol (95% v/v). In addition, the plasma and some related organs (liver, spleen, kidney and gill) were put into the liquid nitrogen and then were transported back to our laboratory and kept in the low-temperature refrigerator with the temperature -80℃. After that, the physiological changes of C. carpio during the sediment flushing process were determined in the laboratory. Comparing with the control group that was collected before the sand discharging, fish collected during the process of the sediment flushing showed significant physiological changes: their gastrointestinal filling index was significantly lower (P<0.001), while the proportion of sand weight in the alimentary canal was much higher (P<0.001); Hepatic total antioxidant capacity (T-AOC) (P=0.036) and catalase (CAT) (P=0.035) activity were significantly higher, but T-AOC activity in spleen (P=0.017) and superoxide dismutase (SOD) activity in kidney were significantly lower (P=0.038); Malonaldehyde (MDA) content in liver (P=0.039) and spleen (P=0.027) were significantly lower. For the experimental fish, the activity of Na⁺/K⁺ATP enzyme in gill was markedly lower (P=0.001), while the content of lactic acid (LD) in serum was much higher (P=0.019) and the activity of lactate dehydrogenase (LDH) in liver was significantly lower (P=0.001). Therefore, the acute hypoxia caused by the rapid flowing water with high sediment concentration may be one of the dominant reasons for "flowing fish" during the sediment flushing process in Xiaolangdi reservoir. To the best of our knowledge, this was the first report to evaluate the acute stress effects on fish during the process of sediment flushing in reservoir. It is a correct decision that this method of regulating the river has been taken into effect and has been contributing to the environment. What we propose is that some index about this process such as the sand content should be paid more attention. So that the environment in the river could be keep balanced while the process itself could also play the important role about sand discharging efficiently and smoothly.