Abstract: The characteristic method is used to calculate the large fluctuation transient process of a diversion power generation system with upstream throttled surge chamber. The influence of the length of headrace tunnel, closing law of guide vanes, roughness of pipeline and moment of inertia of the unit on the value of ideal throttled orifice area in the hydraulic transients are simulated and analyzed. The results show that the larger the length of the headrace tunnel, the moment of inertia of unit and the roughness of pipeline, the smaller the area of ideal throttled orifice. The conclusion has some guiding significance for practical engineering design.