Abstract: The width of section coal pillar directly affects the stability of coal pillar, the difficulty of roadway maintenance and the recovery rate of working face. Aiming at the problems of resource loss and roadway maintenance caused by unreasonable coal pillar size in fully-mechanized caving mining face, taking the geological conditions and mining parameters of a fully mechanized top coal caving face in a mine in Northern Shaanxi as the research object, using the methods of physical simulation, FLAC3D numerical calculation and theoretical analysis, the ground pressure behavior law of fully-mechanized caving mining face, the stability of coal pillar in different width sections and the distribution form of dynamic stress are studied, The deformation and failure characteristics, ultimate strength and instability probability of section coal pillar with different width height ratio are mastered. The results show that the stability of coal pillar has obvious width height ratio effect, the maximum stress and plastic zone width of coal pillar are negatively correlated with the width height ratio of section coal pillar, and the core area ratio of coal pillar is positively correlated with the width height ratio; When the width of section coal pillar is greater than 18m, the nuclear area rate is greater than 65%, the safety factor of coal pillar is greater than 1.1, the probability of instability is less than 45%, and the coal pillar is relatively stable; When the width of coal pillar is less than 18m, the nuclear area rate is less than 65%, the safety factor of coal pillar is less than 1.0, the probability of coal pillar instability is greater than 60%, and the coal pillar is easy to instability. It has been verified in practice in 406 working face of the mine that there is only local slope phenomenon in coal pillar slope and mining slope, and the roadway deformation is small, which ensures the safe mining of the working face. The size of coal pillar can be used as a reference for the study of similar mining conditions.