Abstract: A method for the determination of Hg in underground water by inductively coupled plasma-optical emission spectrometry (ICP-AES) has been developed. Hg(Ⅱ) was separated and enriched according to the coprecipitation of Hg(Ⅱ) by Cu(Ⅱ) and DDTC chelate compound in water. In order to detect the accurate value of Hg, different amount of Cu and DDTC was loaded into Hg standard solution to optimize the added amounts of Cu and DDTC. The lowest detecting concentration of Hg was 0.1 μg/L after selecting optimal analyzing line, correcting background reasonably, and condensing the volume of water from 200 mL to 5 mL with enrichment factor of 40. Certified Reference Materials and practical samples were analyzed and the results were in good agreement with the certified values obtained following national standard methods. The relative standard deviation (RSD, n=12) is less than 10%. Therefore, the proposed method meets the requirements for determination of trace Hg in underground water.