Abstract: The Mengajie hot spring is located in the Menga basin in the western Yunnan. The hot spring emerges along the NW-trending Modaohe-Mandong fault to the west of the Lancangjiang Fault. The subsurface geothermal zone occurs in the neutral-acid intrusive rocks of the Hercynian-indosinian Period. TDS of the hot spring is low (0.31-0.34 g/L), and the hydrochemical type of the hot water is of HCO3—Na type. The hot spring is of low to moderate temperature and is weak alkaline. The F-concentration of the hot spring ranges from 12.8 to 13.2 mg/L, and the H2SiO3 concentration, from 52.5 to 67.6 mg/L. Li, Sr and W are relatively abundant in the hot water. The stable 2H and 18O contents of the spring water indicate that the Mengajie hot spring is of meteoric in origin. The elevation of the hot spring’s recharge area is estimated as about 1 200 m, the temperature of recharge area, approximately 10 ℃, and the temperature of the geothermal reservoir, from 93 to 104 ℃. During deep circulation the hot water is heated and rises along the fault zone after the infiltration of precipitation in the surrounding mountain area.The hot water mixes with the shallow cold water when it rises to the land surface. The proportion of mixing cold water is about 52%-76%, and the circulation depth of hot water is estimated as 3 000-3 360 m.