The Heinitang hot spring is located in the western Yunnan of China and lies in the Tibet-Yunnan geothermal zone and the Tengchong Block. The hot water is of HCO3-Ca•Na type and the F concentration is relatively high (ranging from 3.8 to 5 mg/L). Fossil travertine mounds and cones and new travertine are found at the spring vents. Hydrochemical analyses indicate that the primary factor affecting the deposition of travertine is the concentration of ion pair CaCO30 in the Heinitang hot springs. The Heinitang hot spring shows a positive correlation between the concentration of CaCO30 and the saturation index with respect to aragonite, calcite and dolomite. They are not linearly related, but have an exponential relationship. When the aragonite, calcite and dolomite are in the state of dissolved equilibrium, the corresponding concentrations of CaCO30 are 0.01 mmol/L, 0.008 mmol/L and 0.012 mmol/L, respectively, indicating that the precipitation of travertine can occur when the concentration of CaCO30 is higher than 0.008 mmol/L. In addition, the concentrations of CaCO30 in the Heinitang hot springs decrease gradually with the deposition of travertine. The precipitation of travertine cannot occur when the concentration of CaCO30 decreases to lower than 0.008 mmol/L.