Abstract: This study focuses on determining the stability coefficient of tunnel surrounding rock in ductile strata. By analyzing the environmental and equilibrium relationships of underground rock space,the mechanical transmission and bearing mechanisms between environmental strata and surrounding rock,as well as between surrounding rock and artificial composite structures,were investigated. The direct protective efficiency of surrounding rock on underground space and the original structural characteristics of the surrounding rock were revealed. The composition of the underground space bearing system was constructed,with its load source traced back to the far-field stress of the geological environment. Combining Sakurai's stability limit strain criterion for surrounding rock with the Kastner equation and Duncan-Fama equation,bearing capacity algorithms were derived for natural native structures(NNS)composed of non-dilating and dilating rock masses,respectively. Based on the internal equilibrium mechanism of the underground space bearing system,the load magnitude acting on the NNS was determined,and a method for calculating the safety factor of the NNS was formulated from the resistance-load relationship. Engineering application analyses verified the effectiveness of the proposed method,demonstrating that the safety reserve of the NNS plays an indispensable role in the stability of the underground space bearing system. This study establishes a pathway for advancing the stability analysis and design of rock tunnel space structures from qualitative description to quantitative characterization.