Abstract: This study was conducted to determine whether Ctenopharyngodon idella (C. idella) exhibit a consistent range of preferred flow velocities under complex dynamic hydraulic conditions, In this study, C. idella, a commercially important species in the Yangtze River Basin (total length: 20.5-25.5 cm), was selected as the research subject. Laboratory experiments were conducted in an open recirculating flume with variable-frequency control, where three characteristic flow scenarios were established. Each scenario consisted of three parallel flow areas of equal width arranged along the streamwise direction and represented all possible combinations of three velocity levels(low, medium and high): Condition I (high-low-medium: 0.65 m/s-0.32 m/s-0.47 m/s), Condition Ⅱ (low-high-medium: 0.27 m/s-0.78 m/s-0.48 m/s), and Condition Ⅲ (low-medium-high: 0.23 m/s-0.43 m/s-0.91 m/s).The upstream swimming behavior of C. idella under three complex hydraulic conditions was comparatively analyzed by coupling fish trajectories with background flow fields, and Boolean operations on probability density functions were applied to quantify the preferred flow-velocity ranges of C. idella. We showed that ① fish residence time increased with decreasing flow velocity, indicating a clear tendency to avoid high-velocity areas and prefer low to medium flow regions; ② under all conditions, C. idella predominantly occupied areas adjacent to flume sidewalls where lower velocities prevailed, with preferred ranges identified as 0.40-0.45 m/s and 0.60-0.67 m/s in Condition I, 0.41-0.50 m/s in Condition Ⅱ, and 0.18-0.31 m/s in Condition Ⅲ; and ③ when background flow distributions were similar (Conditions I and Ⅱ), fish exhibited a stable preference for 0.41-0.45 m/s, whereas under distinct flow regimes (e.g., Condition Ⅲ different from Condition I and Condition Ⅱ), preference shifted toward areas with higher probability density and lower velocities. These findings indicate that C. idella consistently prefers low to medium flow regimes, and C. idella have a stable preferred flow velocity under complex dynamic hydraulic conditions. This study provides scientific reference to support the ecological design of fishways and flow restoration strategies tailored to the behavioral responses of native fish species.