This paper addresses the performance requirements on oil-based drilling fluid for drilling of the horizontal well Wanye1HF. The drilling fluid system with density of 1.3g/cm3 and temperature resistance of 120℃ was optimized and evaluated. The effects of 5 main components: emulsifier, auxiliary emulsifier, wetting agent, organic clay and filter loss reducer on the system performance were analyzed to find out the optimum ratio of the 5 components. Then the density gradient and temperature gradient were evaluated for the optimum formula to determine the drilling fluid system suitable for the shale gas block. The system was applied in the field at Well Wanye1HF, and achieved good performance.