To address the problem of low accuracy in empirical sediment discharge ratio formulas caused by the continuous decline in effective reservoir capacity and significant changes in inflow water and sediment conditions during long-term operation of cascade reservoirs, the inflow water and sediment processes in the cascade reservoirs of the lower reaches of the Jinsha River over the next 100 years were designed. The sediment discharge ratios of the cascade reservoirs were calculated using both a one-dimensional water-sediment model and the Siyami empirical formula for sediment discharge ratio. The quantitative relationship between the results obtained by the two methods was analyzed, and the Siyami empirical formula for sediment discharge ratio was subsequently corrected. The results show that the sediment discharge ratio calculated by the one-dimensional water-sediment model exhibits greater volatility than that calculated by the Siyami empirical formula. Moreover, as the operation time of the cascade reservoirs increases, the growth rate of the sediment discharge ratio calculated by the one-dimensional water-sediment model is significantly higher than that calculated by the Siyami empirical formula. The coefficient of determination of the corrected Siyami empirical formula for the Wudongde, Baihetan, Xiluodu, and Xiangjiaba reservoirs is 0.95, 0.96, 0.87, and 0.86, respectively, indicating that the corrected formula has high accuracy.