Hole +304 is a deep coal seam borehole drilled in Huacaotan Kuangqu.The buried depth of main coal seam generally exceeds 800m. The coal quality condition is mainly broken coal, poor cementation, high risk of fragmentation, loosening and collapse. During the use of conventional sodium clay polymer drilling fluid system, complex accidents occur continuously.In this paper, polymer drilling fluid system with easy degradation and strong inhibition is adopted. Water +4% sodium clay +0.05% soda ash +0.3% hydroxyethyl cellulose (HEC)+0.5% degradable polymer (JP)+1% fluid loss agent +1% anti-collapse agent +1% degradable plugging agent +0.5% lubricant + weighting agent (calcium carbonate), apparent viscosity 15~20 mPa·s, fluid loss <5ml, dynamic shear force 8~10 Pa, expansion rate less than 0.1%, using oxidation type gel breaker OPJ 1.0%, The degradation rate reaches 93.5%, the drilling footage is 1077.50m, and there is no collapse, diameter reduction and other complicated situations.