Based on previous geothermal tracing researches by boron isotopes, and by utilizing the state-of-art MC-ICP-MS analytical technique for measuring boron isotope ratios, the mixing processes between deep geothermal water and surface cold water of the Yangbajing (YBJ) geothermal field in Tibet were determined. And then, the authors propose that B is derived from host altered granite according to regional geological features, and the host altered granite should have similar boron isotope ratios to the deep geothermal water. Accordingly, some complicated factors will be introduced into the two end-member mixing model in the YBJ since there is only little difference of boron isotopes between two end-members.