Abstract: Based on the maximum entropy principle,a velocity formula is derived to describe the variation of longitudinal velocity in both the vertical and transverse directions in a open channel with trapezoidal cross-section. The effect on the longitudinal dispersion coefficient of the inhomogeneity in the vertical and transverse directions of the flow is studied.And the conclusions fairly agree with the work of other researchers. A formula for predicting the longitudinal dispersion coefficient in a open channels with trapezoidal cross-section is proposed. According to the formula,the longitudinal dispersion coefficient is determined by two parameters which reflect the inhomogeneity of the velocity distribution and the influence of channel walls. The method on the longitudinal dispersion in this paper is not based on some special experimental results or the field data. Because of the solid theoretical basis,this method can be used as an efficient tool to determine the longitudinal dispersion coefficient in any open channels with trapezoidal cross-section.