Abstract: Accelerating flow generally exists during fish downstream migration passage, which will affect swimming behavior of individual fish. In this study, the swimming behavior of juvenile silver carp (Hypophthalmichthys molitrix) and bighead carp (Aristichthys nobilis) during downstream migratory were tested under two types of accelerating flows (Ⅰ: 22—55 cm/s, Ⅱ: 25—60 cm/s). The objective of this experiment was to quantify the swimming behavior, including downstream pass rate, the way of downstream movement, and the time spent in each area. The preferred flow velocity was measured by the percentage of residence time (F_t). The result showed that the fish performed two kinds of downstream behavior in accelerating flow, which were " direct” and " non-direct”. The direct downstream rate of silver carp and bighead carp were 35.71% and 30% in Ⅰ, and 45.83% and 59.09% in Ⅱrespectively. During the non-direct downstream movement, silver carp spent significantly longer time in the flow less than 30 cm/s under both I and II flow conditions (P<0.05), which indicated that the preferred water velocity of silver carp in the tests was less than 30cm/s. The time spent by bighead carp was significant longer in the flow velocity less than 30 cm/s and 50—55 cm/s under flow conditionⅠ than in flow with other velocity (P<0.05), and under flow condition II, the time was longer in the flow velocity less than 30 cm/s (P<0.05), which implicated that the preferred water velocity of bighead carp in the test was less than 30 cm/s. Based on the results from two types of accelerating flow conditions, it showed that there was significant difference (P<0.05) in the residence time between silver carp and bighead carp in the flow with velocities less than 30 cm/s and 50—55 cm/s, but there was no significant difference (P>0.1) in medium and high velocity conditions. The study demonstrated that the accelerating flow can affect the swimming behavior of individual fish during downstream migration, and there is some interspecific difference in the ways of downstream migration and preferred velocity between silver carp and bighead carp.