Abstract: This study aims to explore the effects of fasting time on the swimming ability of four species of freshwater fish with different body lengths: Mylopharyngodon piceus (black carp), Ctenopharyngodon idellus (grass carp), Hypophthalmichthys molitrix (silver carp), and Aristichthys nobilis (bighead carp). Three groups of experimental fish with different body lengths were selected: short (Group W 4.34±0.21—7.86±0.29 cm), medium (Group S 14.53±0.25—15.6±0.68 cm), and long (Group E 20.29±0.7—23.4±0.51 cm) (n=40). After fasting for 0, 2, 5, 10, and 15 days, the induced flow velocity, critical swimming speed, and burst swimming speed were measured to assess the effects of fasting time and body length on the swimming ability of the fish. The results showed that the induced flow velocity, critical swimming speed, and burst swimming speed were all related to body length. The absolute critical swimming speed and absolute burst swimming speed (cm/s) increased with body length, while the relative critical swimming speed and relative burst swimming speed (BL/s) (body length, BL) decreased with increasing body length. There were significant differences in induced flow velocity, critical swimming speed, and burst swimming speed among the three separate body length groups (W, S, E) for each species of fish (P<0.05). The order of induced flow velocity, critical swimming speed, and burst swimming speed for the four major carp species was black carp>grass carp>bighead carp>silver carp. Under fasting conditions, there was no significant change in induced flow velocity (P>0.05), but both critical swimming speed and burst swimming speed significantly decreased (P<0.05), showing a gradual downward trend. Within the range of 0-15 fasting days, the induced flow velocity of Group W was significantly higher than that of Group S (P<0.05), and both were significantly higher than that in Group E (P<0.01), while the critical swimming speed and burst swimming speed of Group W were significantly lower than those of Group S (P<0.05), and both were significantly lower than those of Group E (P<0.01). The results indicate that compared with Group W, Groups S and E are less affected in swimming ability under fasting conditions, suggesting the release of four major carp species with body lengths over 10 cm. After fasting for 5 days, Groups S and E did not show significant decline in swimming ability. Moderate fasting may enhance feeding ability, but long-term fasting weakens swimming ability, affects feeding, reduces the ability to escape predators, and ultimately lower survival rates. Therefore, fasting should be avoided before release to ensure swimming ability, enhance survival ability in the wild, and increase feeding success rates. In conclusion, the swimming ability of juvenile carp species is affected by body length and fasting, with swimming ability increasing as body length increases, and fasting having the opposite effect. It is recommended to select larger (over 10 cm) and well-nourished fry for release to improve survival rates and the success of stock enhancement projects. The findings of this study provide an important theoretical basis for the optimization of stock enhancement techniques for the four major carp species.