A new method was proposed to measure the thickness of the boundary layer, and the thickness of the boundary layer under different viscosity and different pressure gradient was measured. The research results show that under the same pore throat radius and pressure gradient, the relation between the viscosity of the fluid and the proportion of the boundary layer is linear. The greater the viscosity, the greater the proportion of the boundary layer. Under the same viscosity and pressure gradient, with the increase of pore throat radius, the proportion of the boundary layer decreases rapidly. Under the same viscosity and pore throat radius, with the increase of the pressure gradient, boundary layer ratio decreases and tends to be stable. In addition by drawing the boundary layer thickness ratio chart, the influence of viscosity and displacement pressure gradient on nonlinear seepage is analyzed, and the results further confirme that the boundary layer is the fundamental reason for the nonlinear flow characteristics in tight oil cores.