Resorting to the modified rock mass rating system, the rock mass classification was performed for Hemushan Iron Mine in east China. Ore pillars in the mine were researched based on the thickness-span ratio method, the structural mechanics beam theory and Pu's arch theory. The failure mode and the evolutionary process of destabilization were gained for the backfill stope. The influences of stope size and the consistent coefficient, tensile strength and the internal friction angle of ore rock on the stability of the backfill stope were analyzed due to transforming from block caving to stage backfill. A safety factor equation of the ore pillars was established on the basis of the limit equilibrium method. Results show that the effects of the consistent coefficient and tensile strength of ore rock on the roof critical thickness of the backfill stope are obvious, and the cohesion exerts a tremendous influence on the safety factor of the ore pillars. Finally, the results above were applied to analyze the stability of the backfill stope in Hemushan Iron Mine, the roof critical thickness of the backfill stope and the safety factor of ore pillars at the lump ore belt were obtained, and causes for the failure of the 19th room were understood from these theories.