Based on the geometric characteristics of the velocity profile, a new dimensionless method was proposed and combined with the regression analysis of the flume experimental data, the vertical velocity distribution law and turbulent flow sub-division in vegetated open channel were presented. According to the characteristics of variation rate of velocity gradient, two characteristic heights are determined. New dimensionless velocity and height are defined by the characteristic heights and corresponding velocities, water depth and maximum velocity, and the velocity distribution law is further obtained by the dimensionless experimental velocity data. The results show that the proposed dimensionless velocity is logarithmically distributed near the free water surface, and in the region between the logarithmic law zone and bed, it can be represented by a four-piecewise function.According to the applicable region of each distribution function, the flow sub-division is determined. It is found that the boundary heights do not coincide with the characteristic heights, but are approximately equal to the upper and lower boundaries of the mixing layer.The comparison with other studies illustrates that the proposed dimensionless method, the piecewise functions and the flow sub-division are effective for describing the velocity with good applicability.