Abstract: CCSD-SK1 is an exploration well aimed at probing into the grand geological event and greenhouse climate change in the earth surface system during the Cretaceous period. Drifting sands, fluffy mudstone, water sensitive mudstone and sandstone and alternate layer take the majority in the formation of the well. Research is conducted on the strong rejection capability, binding property and filtration reduction of drilling slurry system, the active mechanism of relevant treating chemicals is analyzed. So the mud system of DFD + LG + Na-CMC is found. In order to test and evaluate the protective capability of the driling fluid, a tester to simulate borehole stability is designed and developed by simulating borehole situations. The breaking pressure caused by the collapse of wellbore is derived from the test, so the anti-sloughing capability of different driling fluids is tested. Field application shows that the drilling fluid system has good rejection capability and binding property. In the situation of coring in bare hole, only using the driling fluid may keep drilling in bare hole up to well completion, and wellbore does not shrink or slough. The technology ensures the field work to run successfully, and also avoids casing to decrease cost.