Abstract: During under-balanced drilling in a shale reservoir, water based drilling fluid can enter the formation due to capillary force when the under-pressure difference is small.Thus, it causes pore pressure change around the wellbore and reduces the stability of the borehole.Becuase of the potential for blowouts, there is a significant need to to carry out the research on the influence law of countercurrent spontaneous imbibition on wellbore collapse pressure in under-balanced drilling.In this paper we discuss how we established a water invasion model of shale reservoir considering the countercurrent spontaneous imbibition in under-balanced drilling which we based on two-phase seepage theory.Our work demonstrates that water saturation near the wellbore will decrease as the under-pressure difference increases.Considering rock strength changes with hydration we set up a stress analysis model near wellbore and studied the equivalent mud density of wellbore collapse pressure of shale.It can be determined from the model that longer drilling times will generate smaller under-pressure differences, and a alarger variation range of water saturation near the borehole will cause lower collapse pressure considering the influence of countercurrent spontaneous imbibition, which is more unfavorable for borehole stability.When other parameters remain constant except for under-pressure difference, a minimum borehole diameter enlargement rate is achieved in the shale.This borehole stability model can provide a theoretical basis for setting a reasonable under-pressure difference and adjusting the drilling fluid density in under-balanced drilling of horizontal shale gas well.