Hole +304 is a deep coal seam borehole drilled in Huacaotan mining area.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 flushing fluid system， complex accidents occur continuously. The polymer flushing fluid system with easy degradation and strong inhibition against collapse was studied， main formula： 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 15~20mPa·s， fluid loss <5mL， dynamic shear force 8~10Pa. After experiments， the expansion rate of coal seam is 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.