Carbonate formations are widely distributed in Guizhou Province with karst caves, karst cracks and joint cracks developing. Karst caves are frequently encountered in deep geothermal well drilling operation. For the purpose of reservoir protection, it is usual to drill against loss circulation. In geothermal reservoir of ZK2 geothermal well of Guizhou, by drilling against loss circulation with low permeability film forming drilling fluid, the cost was reduced and the cuttings were discharged in time. This safe drilling case provides its experience for the application of low permeability film forming drilling fluid in deep geothermal well drilling.