<p>As the sole medium between the support and the roof, the mechanical state of top-coal governs the stability of the “support-surrounding rock” system in steeply dipping coal seam. Therefore, accurately predicting the evolution of the top-coal failure boundary morphology is essential for the stability control of the system. Based on the fully mechanized caving face at Changshanzi Coal Mine as the engineering background. A quantitative analysis of the evolution of the boundary spatial morphology of the top-coal limit equilibrium zone and its dip effect was achieved, and the instability mechanism of the “support-surrounding rock” system in steeply dipping coal seam was revealed. The evolution of the boundary spatial morphology in the top-coal limit equilibrium zone can be divided into three stages: the initial stage, the formation stage of the “asymmetric arc-shaped ribbon-like curved surface,” and the stable stage. The evolution exhibits asymmetry and sequential nature. (1) As the working face advances, the asymmetry of the boundary spatial morphology along the dip direction gradually intensifies. (2) Along the dip direction of the working face, the evolution occurs sequentially from the lower, lower-middle, upper, to the upper-middle sections. Along the strike direction, the evolution proceeds in a top-down order. As the dip angle of the coal seam increases, the evolution of the boundary spatial morphology accelerates, accompanied by an increase in failure depth and a more pronounced asymmetry in the boundary morphology. This offers solid theoretical support for guiding safe and efficient production in similar mine working faces.</p>