To clarify the main driving factors of runoff evolution in the Fuchunjiang Reservoir Basin, the trend and mutation of annual runoff depth, precipitation, and potential evapotranspiration from 1970 to 2022 in the basin were analyzed. The contributions of precipitation, potential evapotranspiration, and underlying surface conditions to runoff evolution were quantified by using the Budyko hypothesis attribution method. The results indicate that both precipitation and runoff depth exhibit a non-significant upward trend, while potential evapotranspiration shows a significant increase. The mutation year for runoff depth is identified as 2012. Increased runoff depth is mainly induced by rising precipitation and changing underlying surface parameters caused by the expansion of impermeable area and the reduction of forest land area, with contribution rates of 51.61% and 52.55%, respectively. The reduction in runoff depth is caused by the increase in potential evapotranspiration, with a contribution rate of 4.16%.