Abstract: In order to solve the problem of wellbore instability in the horizontal section of deep shale gas horizontal wells in Block Weiyuan, a water based drilling fluid with strong inhibition and plugging capacity was developed. Through analysis on mineral composition, reservoir physical properties and wellbore instability mechanism in shale formation in Weiyuan Block, it was believed that the drilling fluid used in horizontal wells of this formation should have good performance in terms of inhibition, plugging and lubricity. The SM-ShaleMud water based drilling fluid suitable for deep shale gas horizontal wells was prepared by optimizing and selecting proper inhibitors, plugging agents and lubricants,, and the lab tests were conducted to evaluate its performance. The lab evaluation results showed that this drilling fluid had good rheological properties, high temperature high pressure (HTHP) fluid loss and low lubrication coefficient; SM-ShaleMud had temperature-resistance up to 140 °C, and presented good performance in inhibiting clay hydration and the generation and extension of fractures; this fluid system had been applied in three wells of Weiye 23 Platform in Weiyuan District, and achieved an excellent application effect. One point had to be stated that it ensured the wellbore stability after being soaked for 67 days, indicating the strong inhibition and plugging capacity. Research suggested that this water based drilling fluid system could solve the problem of wellbore instability in long horizontal section, and achieve a significant application effect, thereby demonstrating a good application prospective.