Abstract: In order to explore the spatio-temporal pattern and the cause of arsenic (As) in Lake Hulun, surface water and sediment samples were collected in spring, summer, autumn and winter. The spatio-temporal distribution and component characteristics of As in Lake Hulun were investigated, and the As source as well as the influences of environmental factors on As distribution in water were analyzed. The results showed that: (1) The ρ(TAs) (TAs: total As) in Lake Hulun was between 6.6 and 87.3 μg/L, with an average value of 47.0 μg/L. ρ(DTAs) (DTAs: dissolved TAs) accounted for 70.6%-99.8% of ρ(TAs), and arsenate (As(Ⅴ)) was the dominant species in the water. The average ρ(TAs) in spring and winter was higher than that in summer and autumn, and the spatial distribution of ρ(TAs) in winter was significantly different from the other three seasons. (2) The w(TAs) in the surface sediment ranged from 1.64 to 15.49 mg/kg, and it showed a decreasing spatial distribution from the northwestern to southeastern parts of the lake. There were relatively high proportions of w(F1) (F1: exchangeable and carbonate bound As) and w(F2) (F2: Fe/Mn oxides bound As) to w(TAs) (31.7% and 30.0%, respectively), indicating a high mobility of these As fractions into water under certain environmental conditions. They were the main source of As in water. (3) Environmental factors such as pH value, freezing period and river inflow affected the spatio-temporal distribution of As. Freezing-caused anaerobic environment at the sediment-water interface and the effect of pollutant concentration were the main reasons for the significant increase of ρ(TAs) in the water of the northwest coast of the lake during winter. The arsenic in the water and sediments of Lake Hulun mainly came from natural sources, and the release of As in the sediments and environmental changes were important factors influencing the spatio-temporal distribution of As in the water.