In the calculation of hydraulic transition process of hydropower stations, for the purposes of regulation and calculation, it is very important to set appropriate values for various pipeline parameters, which is also the primary requirement for safe and regular operation of hydropower stations. In this paper, choosing the right roughness value for the tailrace system in the transition process calculation of a hydropower station is deeply studied by changing the roughness values of the pipelines in front of and behind the surge chamber respectively with the commonly used software designed for the calculation of the transition process. From the calculation results it can be seen that the changes of the roughness values had an impact on the lowest surge level in the tailrace surge chamber, the maximum pressure at the end of the volute and the minimum pressure at the inlet of the draft tube during the discarding load process of the equipment unit when the other conditions remained unchanged. In the calculation of hydropower station transition process, it is suggested that the minimum possible roughness values of the pipelines should be adopted regarding to the perimeter of the surge chamber. These findings can provide some reference for the construction of similar projects.