Abstract: There is very large amounts of computations to compute the response matrix with Monte Carlo method if the incident neutron spectrum is obtained from experimental recoil proton pulse height spectrum. In the paper a analytical method to obtain neutron spectrum is described and a simple computer program has been developed on the IBM-PC microcomputer, This program includes the corrections for second scattering of neutrons and for wall effect of recoil protons. It's tested with T(d,n)neutron source and the neutron spectrum is obtained.