In order to settle the reasonable size of 1304 isolated island of Sunjiagou Mine, combined with FLAC3D numerical simulation software and working condition, pillar width model of 4, 6, 7, 8, 10, 12 m was analyzed, the change of roadway roof, side displacement along empty side and solid coal side was studied. The simulation result showed that pillar width with 6~12 m, the change of displacement of roof and floor was small, it was considered that cola pillar width more than 6 m enabled to handle the mining stress, therefore, the coal pillar width was settled as 7～8 m.