Abstract: To investigate the impact of water flow on fish phototaxis, this study tested the light preference behavior of juvenile grass carp (Ctenpharyngodon idellus), adopting a self-made recirculating channel, under 300 lx light source with three velocity treatments (0, 0.1 m/s and 0.2 m/s respectively) and control group (dark, still water). The results indicated that the flow velocity of 0.2 m/s was able to adequately stimulate the rheotaxis of grass carp juveniles, constraining the dominant swimming direction within ± 20° from the opposite direction of flow. According to the distribution of juvenile grass carps in the test channel with light intensity attenuation, the illumination expectations of test fish at the three given velocity levels were 52.45 lx, 34.62 lx, and 37.86 lx respectively. During the 300 lx trial, fish in still water displayed high distribution on the lateral sides and low distribution in the middle of the channel without a preference for any specific range of light intensities. At the low-velocity level less than the induced value, the overall distribution of juvenile grass carp was similar to that in still water, but farther away from the light source, and more individuals performed " reverse flow backwards” behavior. When the flow velocity surpassed the induced value, the accumulation of fish on the rear side reduced due to the effect of the rheotaxis, and the distribution was more uniform, and the light environment influence is reduced. This study demonstrated that grass carp rheotaxis caused by a small flow velocity, which is slightly larger than the induction value, could affect its light environment response behavior.