In order to study thermally environmental problems caused by rapid expansion of Guilin city in Southwest China karst area in the last 20 years, the METRIC model was improved for suitability of the actual situation in karst cities. Then land surface water and heat fluxes were retrieved with the improved METRIC model from five Landsat images from1994 to 2015. Finally, the spatio-temporal variations of the fluxes were analyzed. The results showed that the land cover types with the values of latent heat flux from the highest to the lowest were water body, vegetation on the sunny slopes of karst hills, ground vegetation, vegetation on the shady slopes of karst hills, buildings and roads, bare soil, and bare rock on karst hills. In contrast, the land cover types with the values of sensible heat flux from the highest to the lowest were bare rock on karst hills, buildings and roads, bare soil, vegetation on the sunny slopes of karst hills, ground vegetation, vegetation on the shady slopes of karst hills, and water body. The temporal variations of land surface water and heat fluxes were influenced by the changes of land covers. The ratio of sensible heat flux to latent heat flux, defined as Bowen ratio, was 1.62 in 1994 as the highest, down to 1.24 in 2000, and gradually rising to 1.51 in 2015. The area proportions of the high sensible heat flux and the low latent heat flux resulted from urban expansion were less than 10.0%. The above area proportion changes resulted in the changes of the area proportions of the middle, low sensible heat flux and middle, and high latent heat flux. The area proportion of the high sensible heat flux was with the highest 10.0% in 1994, went down to 5.4% in 2000, and gradually rised to 9.4% in 2010. But it dropped to 7.1% in 2015. The variation trend of the area proportions of the low latent heat flux is essentially the same with that of the high sensible heat flux. While vegetation fraction ranges between 0.1 and 0.8, it affected the fluxes very obviously. The value of sensible heat flux decreased 8-27 W/m² and the value of latent heat flux increased 8-24 W/m² with the 0.1 rise of vegetation fraction. Vegetation protection on karst hills and ground greening construction are crucial in improving urban heat environment in karst cities.