In order to study more deeply on the influence of the relationship between the impedance ratio of the impedance surge chamber and the size of the pressure channel cross section on the adjustment guarantee calculation results, this paper takes the three water diversion branch common impedance surge chamber as example, and uses the non-steady flow characteristic line method to simulate the pressure diversion system of the hydropower station. Through analysis of surge water level of surge tank, maximum rate of increase of unit speed, and relative pressure increase value of the maximum water hammer at the end of the volute, we concluded that the reasonable range of the ratio of the impedance hole area to the section area of the pressure channel is 28%~46% under the condition that the parameters of the regulation guarantee meet the requirements of the design specification. This conclusion can guide the construction of similar projects.