Abstract: Dayi structure is located in Longmenshan thrust nappe zone of Sichuan basin. The second section of Xujiahe formation is the main formation, a sandstone gas reservoir, with the characteristics of low porosity, low permeability, normal pressure, high heterogeneity and fracture development. And it has mud shale intervals, which causes the difficulties of formation protection. According to the big amount of solid phase and low inhibitive ability of conventional drilling fluids, a formate clay-free drilling fluid system is developed, which meets the need of drilling operation and formation protection in the second section of Xujiahe formation. Through the performance evaluation, it is proved that the drilling system has high solid phase capacity and lubricating ability. The ability of its temperature stability reaches 130 ℃, and the damage recovery rate is 94.49%. It was firstly used in Well DY6 and DY5 in Dayi structure. And the results indicate that the decomposable formate base drilling fluid has high gas formation protection capacity, and solves the issue of wellbore stability. Besides, the drilling fluid is suitable for slim hole drilling. In general, the formate base drilling fluid is a novel technique for formation protection in the second section of Xujiahe formation of Dayi structure.