Utilizing the computational equation of hydraulic transient flow in a long channel of water division of a hydropower station, the paper analyzes and computes all kinds of working conditions of water diversion channel of Hengdan Hydropower Station on Bailong River in Gansu province, and on the basis of the computed results under normal condition, emphatically studies the unsteady transient flow when hydraulic turbine set increases or decreases load. The results of two calculation programs with an overflow side weir and without it are compared, by whcih the highest water level and the lowest water level under the most dangerous conditions are obtained. A suggestion that the canal bottom elevation should be decreased by 0.5 m and the overflow side weir taken off is proposed.