Abstract: Luban Reservoir is the third largest reservoir in Sichuan Province, which plays an important role in irrigation, power generation and flood control. However, concentrations of total nitrogen (TN) and total phosphorus (TP) of the reservoir cannot achieve the water quality standard. In order to address this water quality problem, there is an urgent demand in illustrating the spatial-temporal distribution characteristics of nitrogen (N) and phosphorus (P), assessing the retention amount and efficiencies of N and P, and identifying the critical factors affecting the water quality of the reservoir. The results suggested that concentrations of TN and TP were (0.65±0.22) mg/L and (0.05±0.03) mg/L, respectively. The retention efficiencies of TN and TP by the reservoir were 55.38% and 53.66%, respectively. Irrigation and water transfer contributed 38.11% and 40.85% of TN and TP retention of the total interception of the reservoir, respectively. Apart from the external inputs of N and P in the reservoir, the growth, death and decomposition of algae in the reservoir affected the concentration of N and P and the retention efficiencies of N and P in the reservoir. Therefore, decreasing external inputs of N and P and controlling algae growth in the reservoir would effectively improve the water quality of Luban Reservoir.