Lithium is an important metal resource and new material, and is widely used in nuclear, photoelectric, and other industries. It occupies an important strategic position in economic construction. Geothermal resources refer to a complex, which is integrated by thermal energy, geothermal fluids and their useful minerals that humans can use. There is the considerable reserve of liquid lithium in China. It is of great significance to develop a convenient and accurate determination method ahead of lithium’s exploration, development and utilization. In determining lithium in geothermal water by inductively coupled plasma optical emission spectrometry (ICP-OES), the sample matrix does not produce significant spectral interference, but it does bring serious matrix effect. The high concentration of sodium, potassium, calcium, magnesium and other easily ionized elements in geothermal water has a strong sensitization effect on the determination of lithium. Moreover, the sensitization degree of the four elements varies with each other under different observation modes. Higher sensitization is observed in axial mode than that of radial mode. Furthermore, the apparent sensitization effect by four elements is not a simple superposition of the ionic strength. Experiments also found that the interference degree of sample matrix was greatly affected by atomization flow in both radial and axial observation modes. The lithium recovery decreased with the increase of flow at low flow rates. At medium and high flow rates, lithium recovery increased with the increase of flow. The compositions of batch geothermal water samples vary greatly, so it is not easy to control the accuracy by adjusting the atomization gas flow in lithium analysis. In order to reduce matrix interference conveniently and effectively, a modified matrix matching method was applied in this study. Single sodium chloride was added in standard solutions and geothermal water samples to match different coexisting ions. The detection limit of the proposed method was not significantly higher than that of the traditional method without matrix matching. The detection limits of the proposed method are 0.20 μg·L^-1 (axial) and 0.41 μg·L^-1 (radial). While those of the traditional method is 0.11 μg·L^-1 (axial) and 0.39 μg·L^-1 (radial). The modified method was verified by the analyte addition test and dilution test. The spiked recoveries of three geothermal water samples were 96.5% to 105.6%, with relative standard deviations lower than 2%. The results were consistent with those obtained by inductively coupled plasma mass spectrometry (ICP-MS). The values of dilution samples agreed well with ±2.0% of the original determination. The improved matrix matching method can effectively reduce the matrix interference caused by easily ionized elements. The proposed method in this study is simplified, swift, accurate, and is suitable for batch analysis with different matrix compositions.