Abstract: The sample of geothermal water was filtered on 0.45 μm water system microporous filtering membrane. The initial filtrate in the range of 50-100 mL was discarded and then the sample of geothermal water (500 mL) was collected. Appropriate amount of 50% (φ) nitric acid solution was added for pH no more than 2. Potassium, sodium, calcium and magnesium in the solution were determined by ICP-AES. Analytical spectral lines of 766.490, 589.592, 317.933 and 285.213 nm were chosen for the determination of potassium, sodium, calcium and magnesium respectively. To correct the background and reduce the matrix effect, the methods of background subtraction and the sample dilution were adopted. Linear relationships between values of emission intensity and mass concentration of the 4 elements were kept in definite ranges. Detection limits of the method ranged from 0.000 6 to 0.010 0 mg·L^-1. Mixed standard solution of potassium, sodium, calcium and magnesium with mass concentration of 5, 100, 100, 50 mg·L^-1 respectively was determined, and values of RSDs of potassium, sodium, calcium and magnesium were 0.43%, 1.3%, 0.43%, 0.54% respectively. The proposed method was applied to the analysis of standard material, giving results in consistency with the certified value. Test for recovery was performed by standard addition method, giving values of recovery in the range of 99.6%-102%. Comparative experiments were carried out in the laboratory and it was found that the analytical results of this method were basically consistent with the values given by the traditional methods.