The calculation of mechanical energy loss of flow in open channel is an important part both in hydraulic and fluid mechanics. In this paper the mechanical energy loss is obtained based on the viscous fluid model and the equation of total flow of constant homogeneous incompressible fluid in open channel. The constitutions of mechanical energy loss and their mutual transformation are revealed. Result shows that the mechanical energy loss are resulted from time-averaged velocity gradient and dissipation of turbulent kinetic energy. When the Reynolds number is small, the time-averaged velocity gradient is the main cause of mechanical energy loss; whereas when the Reynolds number is large, the dissipation of turbulent kinetic energy is a major contributor. In conclusion, in shallow and wide open shallow with a Reynolds number of 9.5×10³, the mechanical loss caused by time-averaged velocity gradient is equal to that caused by kinetic energy dissipation.