Abstract: Based on the Forchheimer equation of generalized nonlinear permeability, an in-situ test method for determining the permeability parameters of nonlinear flow in shear zones is proposed. Based on the field tests of Baihetan Hydropower Station, it is found that the values and stability of nonlinear coefficient b and m are related to the filling type and structure of the shear zone. In interlaminar shear zones of C₃ and C₄, m=2, and the value of nonlinear coefficient b is relatively stable with little change. In interlaminar shear zone of C₂, m=0.5, the value of nonlinear coefficient b is relatively stable, but the order of magnitude is much smaller than that of interlaminar shear zones of C₃ and C₄. The reason is that the original pore with poor connectivity of shear zone forms a dominant flow channel under the action of high-pressure water, resulting in significant increase of water volume under the same pressure. Using the nonlinear coefficients obtained from experiments into Forchheimer equation for curves and judged by calculation of influence coefficient β of nonlinearity degree, the nonlinear terms between pressure holes and test holes are absolutely dominant in the process of high-pressure water tests. The movement state of groundwater is nonlinear flow. It is shown that the distance between pressure holes and test holes and the pressure gradient change selected by the in-situ test method are suitable. The test results show that the in-situ test method of nonlinear permeability parameters of shear zones is rigorous in theory and has the advantages of simple test process, easy operation, complete parameters and high accuracy, so it has good application value.