Abstract: The geothermometer is often used to calculate the deep temperatures of geothermal reservoirs in the geothermal system,but the temperatures calculated by different geothermometers are different,which needs to exam the mineral-fluid equilibrium state.The conditions for use of geothermometers in the geothermal system were discussed in the paper by taking the geothermal system in pingdingshan 8th mine as example.The Na-K-Ca diagram and lg(Q/K) figure were used to exam the mineral-fluid equilibrium state of the geothermal system in Pingdingshan 8th mine,which indicated that the chalcedony geothermometer was the most suitable geothermometer for the deep reservior in Pingdingshan 8th mine,and the temperature of the deep reserviors is about 50℃.