Abstract: For the technique of neutron back-scattering, the neutron counts are non-linear and have a tendency toward saturation because of the neutron self-shielding. As a result, the measurement accuracy is reduced and the measurement range is limited. Using a simply model and comparing with experimental data, it is shown that, in the measurement of the thickness of oil dirt, by adjusting the ratio of thermal to epithermal neutrons, the neutron self-shielding is weakened. As a result, the nonlinearity can be reduced and the measurement accuracy and range can be improved.