Abstract: This paper aims to solve the problem of wellbore dynamic instability in ultra deep and dense sandstone formations. The compressibility, inertia, and viscosity coupling effects of pore fluid and hard rock is considered. A stress distribution formula around the wellbore under the condition of wellbore pressure fluctuation is established. A pore elastic dynamic model for wellbore collapse index in ultra deep dense hard rock formations is established. The influence of formation intrinsic frequency and high stress on wellbore instability is analyzed. The pore elastic dynamics model proposed in this article predicts strata that are more prone to instability than traditional models. And strata with high intrinsic frequencies and maximum geostress are more prone to instability than general strata. This paper reveals the mechanical essence of sudden wellbore instability in ultra deep hard rock formations. It provides a scientific basis for adjusting wellbore pressure control parameters during the drilling process.