This study defines reasonable reservoir temperatures and cooling processes for geothermal fluids in three representative high temperature geothermal fields - Yangyi, Yangbajing, and Gulu - distributed along the active geothermal belt of Nimu-Nagchu in south-central Tibet. It uses a combined analysis of hydrochemical compositions, chemical geothermometers, and multicomponent chemical equilibrium analyses of geothermal fluid with an enthalpy vs. chloride plot. There are two geothermal reservoirs in Yangyi and Yangbajing, and both the intermediate and high temperature reservoirs of any one geothermal field are essentially within the same hydrothermal system. The subsurface geothermal fluids from Yangyi cooled mainly by mixing with abundant cold water in the intermediate (163-172 degrees C) and high temperature (192-200 degrees C) reservoir. The subsurface geothermal fluids from Yangbajing experienced adiabatic cooling and mixing with colder water, which formed the high temperature reservoir ZK4001 (255 degrees C) and intermediate temperature reservoirs (164-177 degrees C), respectively, and then emerged on the surface with adiabatic cooling during the ascent. The subsurface geothermal fluids from Gulu ascended to high temperature geothermal reservoirs (211-234 degrees C) mainly cooled by adiabatic boiling or mixing with cooler water. Most of the high temperature fluids mixed with colder water (mixing temperatures range from 149 degrees C to 176 degrees C) during the ascent, and then emerged on the surface as hot springs mainly cooled by conduction. The deep parent fluid of Yangbajing is calculated to have a Cl- concentration of 767 mg L-1 and enthalpy of 1350 J g(-1) (water temperature of 321 degrees C), which agrees well with the maximum temperature measured in well ZK4002 (329 degrees C). The deep parent fluid of Gulu is calculated to have a Clconcentration of 845 mg L-1 and enthalpy of 1290 J g(-1) (water temperature of 307 degrees C).