As one of the most hazardous elements in the environment, mercury poses a threat to humans, plants, and animals. Determination of mercury in water is crucial for environmen_x0002_tal management and toxicological research. In this study, a new method was developed for the determination of ultratrace mercury in tap and lake water based on on-line solid phase extraction (SPE) and ultraviolet vapor generation (UVG), coupled with inductively coupled plasma mass spectrometry (ICP-MS). A SPE column (C18 reverse-phase column) was used to enrich the samples and to eliminate potential interferences from matrix and reagent impurities. Formic acid of 10% (v/v) was used to reduce the mercury ions in water into mercury vapor. Good linearity (R2=0.995) was obtained ranging from 1-1000 ng L-1 Hg2+, with a limit of detection of 0.5 ng L-1 and an RSD of less than 5%. The developed method was validated by spike and recovery experiments and analysis of a certified reference material GBW 10050Prawn. The results show that the SPE-UVG-ICP-MS method is sensitive, fast, precise, and cost-effective for mercury analysis, especially for natural water samples.