The analysis of water and sediment data with long time series is helpful for the scientific evaluation of flow and sediment reduction effect of watershed-scale soil and water conservation, and provides an effective method for the scientific evaluation of regional soil and water conservation effectiveness. The Pingjiang River Watershed in the upper reaches of Lake Poyang Basin, which is the typical watershed with the most concentrated and longest ecological protection engineering construction in the red soil region of southern China, was selected as the study area. Based on the daily rainfall, flow, and sediment concentration data of 1975-2014, pre-whitening Mann-Kendall test, Theil-Sen trend analysis, Pettitt's test, and comparative method of the slope changing ratio of cumulative quantity were conducted. Variation characteristics of flow discharge and sediment load were analysed, major influencing factors were identified, their contribution rates to flow discharge and sediment load variation were quantified, and effects of long-term soil and water conservation on flow discharge and sediment load were revealed. The results showed that: (1) From 1975 to 2014, sediment load of the whole year, flood season, and non-flood season all decreased significantly, with the yearly decreasing rate of 2.38×10⁴, 1.75×10⁴, and 0.44×10⁴ t/a, respectively, and the mutation of flood sediment load occurred once the soil and water conservation project began, while the mutation of the whole year and non-flood sediment load didn't happen until most of soil and water conservation projects accomplished. (2) Long-term soil and water conservation in the watershed significantly reduced the sediment load in Pingjiang River, yet had no significant effect on runoff discharge. During both the key control period and post-control period, the contributions of ecological protection projects, which mainly focused on soil and water conservation, contribution rate to the reduction of sediment load were more than 94.0%, while the contribution of ecological protection projects to the reduction of sediment transport in flood season was more than that in non-flood season.(3) Long-term ecological protection projects based on soil and water conservation resulted in a decrease of 59.3%, 60.7%, and 55.7% for sediment load of Pingjiang River during annual, flood, and non-flood period, respectively. (4) After excluding the impact of large-scale hydraulic engineering construction projects, especially large and medium-sized reservoirs construction, the effect on flow and sediment reduction in the watershed scale of soil and water conservation could be scientifically evaluated based on the analysis of long-term flow discharge and sediment load data in the red soil region of southern China. The results can be used as reference for the evaluation of the effect on soil and water conservation in the watershed scale of soil and water conservation in the red soil region of southern China, and are expected to provide scientific basis for the layout and management of national ecological protection and construction projects.