Abstract: With unchangeable water pressure method,big size and low permeability soft rocks have been systematically tested with the new equipment called High Pressure Permeability Testing Machine in Lab.The penetration water pressure fluctuation scope only is 0.01 MPa,and the seepage exit water volume testing precision is 0.05ml.According to linear relationship of water volume and time,seepage quantity can be accurately calculated.The test results demonstrate that the non-Darcy flow phenomena are applicable for low permeability rocks.On the basis of the linearity seepage characteristics of non-Darcy flow,the quasi-threshold pressure gradient has been calculated with three methods.And it is proved that Averaged Two Times Method is the best one through examples.