In recent years, Extended reach well (ERW) drilling technology is widely used in the offshore oil & gas fields in order to reduce the number of the drilling platforms. As it has notable characteristics such as the high deviation angle, large horizontal displacement and long open borehole interval the borehole stability increases the drilling risk and cost dramatically. To research the ERW borehole stability, including mechanical model, shale hydration test and the effect of circulating pressure loss in this paper, rock mechanics theory and hydraulics principle were comprehensively applied . The results show that, the safer drilling azimuth of the ERW in normal fault lies in the minimum horizontal principle stress direction; hydration radius increases with the passage of time, and the hydration collapsed rock has important influence on cutting beds and circulating pressure loss in annulus; the upper limit value of safety mud density decreases with the well depth increases, and when it deceases to the equivalent mud density of collapse pressure, the limit depth of the ERW is obtained. The research results provide relevant guidance on the ERW drilling, which can be used to determine the upper and lower safety mud weight limits, the best well trajectory selections and the well structure design.