Based on the water quality data of the inflow into Poyang Lake and the land use data of the basin from 1990 to 2020, the driving characteristics of land use changes in the basin on the water quality of the inflow into Poyang Lake were analyzed. The results show that the concentrations of total nitrogen and total phosphorus in the inflow rivers have significantly increased, with the increase in total nitrogen being particularly prominent. After 2010, the average concentration of total nitrogen increased by more than 83.2% compared with the previous period, and the total phosphorus exceeded the Class III water standard of lakes and reservoirs in 90% of the time after 2010. The conversion of cultivated land to construction land is the dominant type of land use changes in the basin, and 90% of the expansion of construction land comes from the occupation of cultivated land. The average annual growth rate of construction land in the Ganjiang River Basin is 67.8 km², and the transformation intensity is the highest in the downstream area. Construction land and cultivated land are the key land use types that lead to the deterioration of water quality in the Poyang Lake Basin. The transformation of cultivated land to construction land in the downstream area contributes 25.9% to the increase in pollutant concentrations. The correlation coefficient between the change in total nitrogen concentration and construction land is as high as 0.819, while the correlation between total phosphorus and land use is relatively weak due to the influence of particulate migration.