Abstract: Borehole stability is one of the most important factors for successful drilling of extend-reach well.Based on the comprehensive analysis of logging data,geological information and laboratory tests results,this paper studied the pore pressure,in-situ stresses and rock strength of Liuhua 11-1 oil field,analyzed the borehole stability during the drilling of shale and limestone payzones for extended-reach wells,and calculated the safe mud density window for the extended-reach wells.Field application showed excellent results,which provide a quantitative base for the drilling design of extended-reach wells in the field.