Abstract: A kind of environmental-friendly water base drilling fluid, PRD drilling-in fluid, was developed to deal with tripping difficulty and carrying cuttings in extended reach wells and horizontal wells. PRD drilling-in fluid composes of several highly biodegradable natural high polymer additives. The BOD₅/COD_Cr ratio of PRD system is 0.42. The dissolved oxygen decay evaluation showed that the two natural polymers, PF-VIS and PF-FLO were readily biodegradable, and PRD drilling-in fluid’s biological toxicity test met the biological toxicity limit requirement for the first level seas. Low shear rate viscosity (LSRV) was up to 60 000 mPa?s so that the system could carry and suspend cuttings with no cuttings bed and the core return permeability was more than 90%, which caused tiny damage to reservoir. The PRD drilling-in fluid was successfully applied in Bohai Bay, East Sea, South Sea and Xinjiang Oilfield, with greater oil and gas production than other drilling fluids systems.