The geological conditions were extremely complicated with poor wall stability and severe sloughing problems in WFSD-4S hole of Wenchuan earthquake fault scientific drilling project. To solve the above problems, the drilling fluid system of bi-polymer anti-sloughing system was adopted in the drilling construction, which had excellent properties of stability, rheology, cuttings carrying and anti-sloughing, and presented effective application result in WFSD-4S hole drilling construction. Through optimizing the formula of drilling fluid and rationally adjusting and maintaining the performance parameters, the technical difficulties in WFSD-4S hole drilling were successfully solved, which laid a solid foundation for smooth completion of drilling construction.