A model for optimization of the bottom shape of the rectangular orifice surge tank in the tail tunnel used by three units is studied in this paper.Measured are the head losses for three types of bottom shape and the changes of head loss under different loads.The results demonstrate that the curve bifurcation model is the best and its head loss is 50% less than that of the non bifurcation model.The bottom shape recommended can increase the efficiency of power generation significantly.