Yanchang oilfield shale gas reservoir is deeply buried with complex geological conditions, foliation structure development, poor physical properties of rock and shale easily expanses and disperses. Because of the coexistence of pores and cracks, the characteristics of the shale reservoir are worse than the sandstone oil reservoir, such as the particle movement damage, reservoir stress sensitivity damage, water blocking damage and so on. In view of Yanchang oil shale reservoir is too dense, the permeability measuring can not be made by conventional methods. The artificial fracture method was used to determine the pollution degree of drilling fluid to characterize the damage rate of drilling fluids to the reservoir. Measured by the artificial fracture method, the return pollution permeability rate is up to 91.060% with the use of white oil-based drilling fluids system in Yanchang oil field. It is preliminarily confirmed that this drilling fluids system has good effect in reservoir protection and can be further promoted and used in Yanchang oil field.