Algal bloom, caused by eutrophication, is one of the major ecological disasters threatening the safety of drinking water reservoirs. Investigating the driving factors of algal bloom in drinking-water reservoirs is needed for preventing and controlling algal blooms. In this study, environmental factors in a typical drinking-water reservoir in Lake Poyang Basin were analyzed before and during algal bloom to reveal the driving factors of algal blooms. In addition, isotopic tracing technique was used to trace the main sources of pollutants. The results showed that: (1) Algal blooms in the drinking-water reservoir were mainly caused by the increasing concentration of total nitrogen (TN) from 0.77 to 1.57 mg/L, which exceeded the internationally recognized threshold of TN (0.5-1.2 mg/L). High temperature in summer (32℃) and low reservoir capacity further promoted the growth and bloom of algae. (2) Before algal bloom, the main occurrence of TN in the reservoir was nitrate (NO₃^--N, accounting for >90% of TN). In contrast, the main occurrence of TN was organic nitrogen (accounting for about 86% of TN) during the algal bloom period due to algae assimilation. (3) Nitrogen sources were identified based on nitrogen and oxygen isotopes of nitrate. NO₃^--N in the reservoir was mainly from nitrogen fertilizer in the upstream paddy field (64.45%), followed by water and soil erosion in the upstream mountain area (14.08%), atmospheric precipitation (12.35%), and rural domestic sewage (9.12%). Therefore, controlling nitrogen fertilizer in the upstream paddy field is a priority strategy for controlling TN in the reservoir.