Abstract: To exploit the coal under high stress water, the characteristics of relative aquifuge of top Ordovician in Longwan well field of Qinshui Coal Area were discussed through hydrogeological drilling, hydrogeological test, flow logging, salt logging, complete stress-strain test. The results were as following: Longwan well field developed ancient weathering crust; the water yield property was weak; the permeability was low; the thickness of the relative aquifuge was more than 40 m, and could reach 120 m above.