Based on the theory of transient flow, the elastic water hammer characteristic line method was used to establish the reasonable boundary conditions，and the numerical simulation analysis was made on the sensitivity of section change of surge chamber in long tailrace tunnel. and detailed study the effect of surge of water level in surge chamber maximum rise rate of disturbed unit and pressure at end of volute under large fluctuation condition and unit output swing of hydropower station on the section area of Tailrace surge chamber under hydraulic interference condition. On the same characteristics condition, under the condition of large fluctuations, when the surge chamber area is 615.752m2, minimum surge level water level reached 2218.97m, the end of the volute pressure reaches 269.17m (pressure on 24.48%), maximum speed rise rate reached 42.19%, which meet the request of the regulation guarantee calculation. Under hydraulic disturbance condition, incorporated into the same grid, when the surge chamber section area increased, the unit output swing will decrease. For large power grid, the relative difference of output swing have smaller minor change to small power grid. Besides, the relative difference of disturbed unit differ is 0.08% when double units load rejection, and the relative difference of disturbed unit differ is 0.03% when just single unit load rejection. Considering the calculation result and the engineering economy of the surge chamber synthetically, it is reasonable to recommend that the sectional area of the tailrace surge chamber of the power plant be 615.752m2.