Abstract: According to different combinations of mining methods and water body positions in deep coal seam,three basic structural patterns were created and main basis of setting up reasonable waterproof coal pillar size for each pattern were proposed.The mechanics model of the stability of waterproof pillar was established,and formulas for critical value of stable length of waterproof pillar were derived.Based on different conditions for each coal mine,the optimization design theory on setting up reasonable waterproof coal pillar size was proposed.The above optimization design theory was applied to a mine in Tangshan.Results showed that the reasonable size of waterproof pillar could be 59.3m.Actually the steep mining face was safely and successfully exploited with 60m height of waterproof pillar,indicating that the theory could be useful for practical projects.