Physical experiments are carried out in a rectangular flume with a length scale of 20 models, and the velocity is measured in vertical particular depth. First, the velocity distributions in vertical direction are initial fitted by curves of the parabolic and logarithmic formula based on the theory of open channel flow and analyzed, the strongly affected zones, moderately affected zones and weakly affected zones are classified from the effect degree. Then, the formula coefficients are deduced by the method of Boxplot fitting, and the formulas of vertical velocity distribution are obtained with parabolic equation in strongly and moderately affected raft zones and coefficients of the former larger than the latter; however, the logarithmic equation is more suitable to weakly affected zones. Finally, the analysis on effect of aquiculture density leads to the conclusion on the parabolic coefficients and the range of logarithmic coefficients increase with the density increasement.