Abstract: In order to clarify the mechanical reasons of loss stability of borehole wall off ractured rock formations and determine the rational drilling fluid density necessary for stabilizing the borehole wall of this kind formations, this paper analyzed the mechanism of loss stability of these fractured formations. The limit value of drilling fluid density for avoiding the borehole wall instability and the hydrostatic column pressure model were also determined in this paper. The influences of cutting sections of the formation rocks on the borehole wall, radial intersection angles and inner friction angles of the structural faces on the degree of loss stability and limit hydrostatic column pressure of the drilling fluid were also calculated and analyzed.