Abstract: The coupled mathematical model of seepage and the horizontal well flow is established based on the motion and continuity equations of the horizontal well flow with the lateral inflow and outflow, and used to calculate numerically the mixed head loss of two flows. The effect of the frequent modified method of friction coefficient on the calculation results is analyzed. The results show that there exists little effect on calculation results as for the laminar or smooth turbulent flow, but great effect as for rough turbulent flow. Furthermore, for the laminar or smooth turbulent flow, a new method for determining the demarcation point of flow pattern is proposed to judge the pattern of the horizontal well flow. And the calculation results show that this method is reliable and reasonable.