Abstract: The problem of borehole instability in Triassic of Anyue region has been recognized as a major difficulty for the exploration and development of the gas field. The key of successful drilling of complex lithological formation was to use drilling fluid with appropriate density.This paper analyzed and researched the unsteady factors of Anyue region Triassic complex formation and the difficulties to the implementationof drilling fluid technology, improved the compatibility of the drilling fluid and optimized polysulfonate drilling fluid system as the appropriate drilling fluid. The drilling fluid system has good characteristics in rheology, wall-cake-building ability, oiliness and inhibition. The case study shows that the drilling fluid is of perfect effect on such successful drilling, underground safety and hydrocarbon reservoir protection