Abstract: In normal days of no rainfall,the hydrochemical variation of the exit of a karst ground river system has a remarkable diurnal periodicity on a short time scale（about 10 days） and the periodicity can be identified using continuous power spectrum analysis and cross spectrum analysis.In this article,taking the outlet of the Banzhai karst ground river system in the Maolan Nature Reserve as an example and using the method of spectrum analysis,the authors investigate the data of meteorological factors and hydrochemistry which were continuously monitored for about ten days of no rainfall in the middle October in 2007.The results indicate that the hydrochemical variation of outlet of the karst ground river system covered with virgin forest shows a remarkable diurnal periodicity.The water pH rose,whereas CO2 partial pressure,electric conductivity and calcite saturation index drop during daytime when the air temperature rose.The response of water pH to meteorological factors and the response of CO2 partial pressure to water temperature are most sensitive.