In this paper, the trace amounts of selenium, arsenic and mercury in seawater were quantitatively detected by coupling hydride generation (HG) device with solution cathode glow discharge optical emission spectrometry (SCGD-OES). Instrument conditions were optimized, and the optimal instrument conditions for quantitative analysis of selenium, arsenic and mercury were determined: 5% HCl as the carrier acid for subsequent experiments, 1.5% NaBH4 as the reducing agent, the SCGD parameters of the electrolyte, discharge voltage, and flow rate were maintained at pH 1 HCl, 1 060 V, and 2.2 mL·min-1, respectively. Element wavelengths of 203.4, 228.8 and 253.7 nm were selected as the analytical lines of selenium, arsenic and mercury, and the mixed standard solutions of selenium, arsenic and mercury were determined under the optimal working conditions simultaneously. The mass concentration of selenium, arsenic and mercury is linear with the emission intensity in the range of 2~100 μg·L-1, and the linear correlation coefficients are 0.999 2, 0.999 4 and 0.998 5, respectively. The detection limits were 0.54, 0.92 and 1.91 μg·L-1. The relative standard deviations of the signal values of selenium, arsenic and mercury are less than 3% at a concentration of 0.1 mg·L-1. Compared with SCGD-OES, the detection limits of selenium, arsenic and mercury are reduced by three, four and two orders of magnitude, respectively. Soil reference material GBW07405 was selected to verify the accuracy of the results of the coupling instrument, and the measured value was consistent with the reference value. The method has been applied to the analysis of trace selenium, arsenic and mercury in seawater samples from the Yellow Sea coast of China and the analysis results are in agreement with those of ICP-MS. The recoveries of standard addition are between 94.9% and 105.3%. HG-SCGD-OES enables highly sensitive on-line quantitative detection of trace amounts of selenium, arsenic and mercury in seawater.