The downstream surge chamber of a certain pumped storage power station adopts impedance with upper chamber layout. The tailrace tunnel is connected with the connecting pipe through right angle bifurcated pipe and bend. The layout type is special and the water flow conditions are complex. In order to study its hydraulic characteristics, a three-dimensional flow field mathematical model of the connecting pipe at the bottom of the surge chamber was established, and the variation law of hydraulic characteristics under different diversion ratio and confluence ratio was studied. The results show that for the resistance loss of surge chamber with complex bottom flow, the three-dimensional numerical simulation can achieve better simulation effect, and the calculation results are in good agreement with the theoretical analysis results. Because the elbow at the bottom of the surge chamber, the sudden expansion and contraction between the connecting pipe and the big well will cause additional hydraulic losses, its head loss coefficient is greater than that of the conventional impedance surge chamber, and the flow coefficient is smaller than that of the separate impedance hole.