Extended reach wells have been widely implemented due to the demand of oilfield development and the contradiction of sea use. Due to the lack of overburden pressure and short sedimentation time, shallow formation tends to be poorly bonded, and it is hard to get a real core from underground. The high content of Smectite makes it active in water-rock interaction. Rock on the wellbore may absorb the free water from the drilling fluid and expand, making the wellbore unstable. Extended Reach Wells face a greater risk of wellbore instability due to large horizontal displacement, long angle holding section and long construction period. To reveal the stability of Extended Reach Wells, the well logging data is processed to calculate formation parameters by physical principle and empirical equation, the core experiments are conducted for calibration, and the lost circulation test is carried out to get in-situ stress. By analyzing and describing the geological characteristics and correcting the stratigraphic profile parameters, the wellbore stability of extended reach wells is predicted based on the mechanical analysis, giving the equivalent density window of collapse pressure, leakage pressure, and fracture pressure. In this way, the risk section can be pointed out and the construction of extended reach wells in Bohai oilfield can be supported.