Abstract: In order to reduce the lower limit of neutron detection energy effectively and solve the problem of sharp decline above 5 MeV at energy response in the long counter, the Monte Carlo simulation was performed in this paper, which was used to analyze the influence of various structural parameters on the energy response systematically. A long counter was designed and optimized by adding moderator with different hydrogen contents, neutron absorption material and compensation material. The result shows that energy response of the low energy neutron can be improved by using polystyrene containing lower hydrogen content, the problem of the high energy response in partial can be solved by using Cd as neutron absorption material, and Pb can be used to improve the energy response of the high energy neutron as compensation material. The maximum relative deviation of calculated response is about 10.1% within the energy between 1 eV-15 MeV for the designed long counter.