Abstract: Hydraulic flushing and permeability improving technology has been widely used in gas extraction in soft coal seams. However, the strength of soft coal is low, so creep deformation is easy to occur, resulting in serious borehole shrinkage and thus affecting gas extraction efficiency. Aiming at the shrinkage problem of hydraulic flushing borehole, firstly, we derived the 2D visco-elastoplastic stress-strain analytical solution of coal around the hydraulic flushing borehole based on the basic mechanical principles such as plane strain assumption, strain softening-expansion characteristics of coal, fractional Maxwell model and Mohr-Coulomb criterion, and then constructed the dynamic evolution model of borehole radius. Based on this model, the dynamic shrinkage mechanism of hydraulic flushing borehole was numerically analyzed by adopting the COMSOL Multiphysics numerical analysis software. The results show that the radial stress and tangential stress of the coal around the borehole do not change obviously with time after the borehole is constructed, while the radial strain increases continuously, resulting in a gradual decrease in borehole radius under the effect of creep. When the borehole was just constructed (0 d), the radial strain in the coal at the borehole wall was only 8.59%. However, when the borehole was constructed for 120 days, the radial strain in the coal at the borehole wall increased to 35.17%, the borehole shrinkage amplitude reaches 68.2%. At the same time, the borehole shrinkage is greatly affected by parameters such as elastic modulus, viscosity coefficient, fractional order, expansion coefficient and initial ground stress. Specifically, the smaller the elastic modulus and viscosity coefficient, the higher the fractional order, the expansion coefficient and the initial ground stress, the larger the radial strain of the coal around the borehole, and the more serious the borehole shrinkage.