The Nansi Lake has been seriously affected by long-term intensive industrial and urban activities. The objectives of this study were to determine the content, distribution, and ecological risk of arsenic and investigate the geochemical relationships between arsenic forms and sediment mineral phases of the Nansi Lake. Twenty samples of surface sediments were collected and analyzed for arsenic contents and chemical forms. Results indicated that total content of arsenic in the sediment samples averaged 13.45 mg/kg and ranged from 8.27 to 21.75 mg/kg. The arsenic was mostly associated with iron oxides (67.3%), followed by the association with the residual fraction (19.2%). In addition, total content of arsenic was positively correlated with the organic matter and iron contents in the sediment. The molar ratios of iron oxide bound arsenic content to iron content are lower than the maximal molar ratios of arsenic to iron for natural hematite, magnetite, and goethite. The total content of arsenic in the sediment samples was usually higher than threshold effect concentration of 9.79 mg/kg, but lower than probable effect concentration of 33.0 mg/kg for arsenic in freshwater sediments. Adverse effects or toxicity to the aquatic organisms, caused by arsenic in the sediments of the Nansi Lake, will likely occur at these levels of arsenic contamination.