The leakage flow characteristics of the experimental brush seal with zero clearance and clearance at different pressure ratios and rotational speeds were numerically investigated using ReynoldsAveraged NavierStokes（RANS）solutions based on the nonlinear Darcian porous medium model.The corrected formula for the blow down effect of brush seal was presented.The numerical predicted leakage flow rate of the clearance brush seal was in well agreement with the experimental data with consideration of blow down effect,which validated the corrected formula for the brush seal blow down effect.The research results show that the clearance between the bristle tip and the shaft results in the decrease of the pressure and relatively obvious radial pressure difference in the bristle pack of brush seal,and the radial pressure difference leads the blow down effect of bristle pack,and then leads to the reduction of effective clearance of brush seal.The effective clearance of brush seal is decreased with the increasing of pressure ratio due to the blow down effect.The prediction accuracy of the leakage flow rate of brush seal can be improved significantly with consideration of blow down effect.The leakage flow rate of the clearance brush seal is larger than that of the zero clearance case at the same pressure ratio and rotational speed.The leakage flow rate of brush seal is increased with the increasing of the pressure ratio,while the rotational speed has less effect on leakage flow rate in the presented rotational speeds range.