The MK-3 well was drilled for investigation and evaluation of potash resources in the Mesozoic salt basin in southwestern Yunnan with the design depth of 2700m, full hole coring and the core diameter not less than 80mm, the core recovery more than 80% over the non-ore bearing section, and more than 95% over the ore-bearing section. The technical requirements were demanding and it was difficult to drill. During drilling, problems such as stratum collapse, water gushing, stone falling and salt rock dissolution were encountered. The drilling task was successfully completed by adopting measures such as magnesium chloride saturated brine drilling fluid, coring with HXY-9B drilling rig, reaming with TSJ-2000 water well drilling rig, multiple tiers of casing for staged sealing off. The well was completed at depth of 2701m with average core recovery rate 90%, and 98.65% over the ore-bearing section. The final hole diameter was 127mm, and the core diameter was 81mm. Geophysical well logging indicated that each index has met the design requirement. It breaks the record of drilling depth by CHD127 standard wire coring drill pipe for “P size” in China.