The determination of reasonable coal pillar size in isolated island working face with large mining height is one of the key factors affecting coal recovery rate and safe mining. Taking 15115 isolated island working face with large mining height in Huangyanhui Mine as the research object, a coal pillar superposition-loaded mechanical model was constructed for the coupling effect of thick hard rock deflection-coal mass collaborative compression and hard rock cantilever additional stress, and the reasonable width was determined. The main conclusions are as follows: The loading at the roadsides is mainly determined by the effect of thick-hard rock stratum deflection and compression deformation of coal by its own weight, while the loading at the goaf side comes from the additional stress of the lateral cantilevering rock stratum. The load and ultimate bearing capacity of the coal pillar show a reverse trend in the width, and the minimum width is 9.4 m. The sensitive point of the equivalent fracture angle on reducing the load strength of the coal pillar is determined to be 82°. In the simulation, the proportion of plastic failure range and the degree of coal mass expansion in the increasing of the coal pillar width decrease. The stress concentration in the middle of coal pillar tends to stabilize, and the overall bearing performance is enhanced. Considering the above, the column width is determined to be 10 m. Engineering practice has proved that, after fracturing on both sides of the roadway, the rock fracture penetration weakens the integrity of the thick and hard rock strata. 10.0 m coal pillar retains the 2.0 m elastic zone in the middle, and the convergence of the roadsides and the roof-floor is controlled within 520 mm, which effectively achieves the safe protection of the island face roadway.