Abstract: In response to the issues of boreholes shrinkage crossing expansive mudstone in coalmine,we discusse the application of polyamines drilling fluid in drilling hydraulic discharge system. Based on linear expansion experiment and uniaxial compression experiments, mudstone expansion and strength characteristics were studied in different drilling fluid. The deformation of expansive mudstone boreholes in different drilling fluid is also analyzed and studied by theoretical calculation and similarity simulation experiment. Results show that polyamine drilling fluid can reduce the moisture content of mudstone, reduce the hydration expansion, and inhibit mudstone expansive deformation. Aluminum salt complex is added in the drilling fluid, which can significantly improve mudstone mechanical properties and reduce the weakening effect of mudstone strength in the hydraulic discharge process. Polyamine drilling fluid system can effectively inhibit mudstone boreholes shrinkage,and shrinkage rate is reduced by 21%.