The rock formations encountered in geothermal well drilling in Guizhou are mainly limestone, dolomit and some unstable formations such as sandstone, mudstone and coal seam. To meet the demands of keeping borehole stability and matching with screw motor drilling tool, potassium-ammonium polymer drilling fluid was optimized. The drilling fluid formulation was obtained through orthogonal tests, and a series of performance tests were conducted in laboratory. Test results show that FT-1 and QS-2 have a synergistic effect on effectively improving mud cake quality, and the friction coefficient of the mud system can be reduced to less than 0.2 by adding 1% of white oil into the system. The new drilling fluid system has good rheological properties with strong inhabitation ability and good lubricating property. Compared with solid-free polymer drilling fluid, the new novel drilling fluid has stronger borehole cleaning ability and lower risk in loss of circulation, the ROP can be increased by 129%.