The fishway in hydraulic facility provides artificial passages for the fish migration in the natural ecosystem． However，different swimming behavior of fish will be exhibited with respect to different characteristics water-flow velocity of various fishway inlets． In this paper， grass carp and cyprinoid are selected as the model fish based on the light sensitivity of most common juvenile cyprinid in Chinese rivers． In order to identify the favored water-flow velocity of model fish，the four characteristic velocities of fish activity ( respondent speed，critical speed，persistent speed and onrushing speed) are defined as the experimental observation indices． The correlation between the carp body length and the characteristic water-flow velocity of the juvenile cyprinid is tested by using the self-developed bionic device to imitate the water -flow through fishway． The formula between the characteristic water-flow velocity and