This paper explored the influence of extreme rainfall on flow and sediment in the southern red soil region based on the daily rainfall, runoff and sediment discharge data of from 1984 to 2020, in the Upper Ganjiang Basin of the Lake Poyang system in Jiangxi Province. The 95^th percentile method was used to isolate extreme rainfall, and the minimum inter-event time method was used to split rainfall events. The Pettitt test and linear regression method were used to analyze the abrupt change and influencing factors of flow and sediment. The results showed that: (1) The period of extreme rainfall events was the critical period of sediment output in the basin, and the contribution of extreme rainfall to sediment was 85.58%-87.79%, while the contribution to runoff was 38.33%-43.42%. (2) Under the extreme rainfall scenario, the annual runoff decreased from 209.21×10⁶ m³ in 1984-1995 to 165.23×10⁶ m³ in 1996-2020, while the annual sediment increased from 3.65×10⁴ t in 1984-1995 to 12.8×10⁴ t in 1996-2020, and compared with 1984-1995, the percentage of runoff and sediment generated by extreme rainfall in 1996-2020 has a decreasing trend, respectively showed from 43.42% to 38.33% and from 87.79% to 85.58%. (3) Rainfall was the main factors influencing flow-sediment changes in the basin under extreme scenarios, with standardized regression coefficients of 0.39-0.57. The flow-sediment changes under the normal rainfall scenario was mainly influence by the antecedent rainfall, with standardized regression coefficients of 0.10-0.29. (4) For extreme rainfall only, the standardized regression coefficients of rainfall on runoff and sediment increased from 0.33 to 0.36 and from 0.12 to 0.17, respectively, from 1996 to 2020 compared with 1984 to 1995. It suggested that the effect of antecedent rainfall on flow-sediment change was more significant in the period of “reduced runoff and increased sediment”. Therefore, when formulating basin management strategies, attention should be paid to the influence of extreme rainfall on the flow-sediment changes to ensure that the measures are feasible.