Abstract: CB6D-P3 well is the first horizontal well drilled in the sea area of CB-6D to explore the buried hill reservoir. In the upper formation, natural-macromolecule polymer drilling fluid was used. In the horizontal formation, the solid free seawater drilling fluid was used. The two drilling fluids met the requirement for safe drilling, reservoir protection and marine environment protection. Laboratory tests and field using show the polymer drilling fluid has good inhibition, good lubrication, and the solid free drilling fluid has good rheology, high tempertature tolerance. And can take the cuttings out good. Now two wells have been drilled using these two fluids.