Abstract: SLCORE-I well is the first Cretaceous continental scientific drilling well in the world.It's the most im-portant engineering method of acquiring deep earth sciences research samples for "Major Geological Events in the Earth Surface System during the Cretaceous and Greenhouse Climate Change" project(2006CB701400),which belongs to the National Basic Research Program(also called 973 Program).It is made up of main well(also called north well) and south well.The continuous coring section of the main well is from 164.77 to 1792 m.The author analyzes the drilling fluid technical difficulties of the main well and processes lab optimization test of drilling fluid systems.To overcome the difficulties in the core drilling process of loose sandstone,water sensitive mudstone and friable hard mudstone,not only different drilling fluid systems such as PAM drilling fluid,DFD-LG-CMC drilling fluid and DFD-NH₄HPAN-SAKH drilling fluid,but also feasible controlling technology of viscosity and drilling fluid technology of decreasing circulating pressure loss are used,which explores one successful way of using low cost drilling fluid to proceed similar scientific drilling project.