Abstract: The first gamma total absorption facility (GTAF) in China has been constructed in China Institute of Atomic Energy, which will be used to accurately measure the neutron capture cross section by prompt gamma method. A neutron source is established by ⁷Li(p, n)⁷Be reaction using pulsed proton beams generated by HI-13 tandem accelerator of CIAE. In order to effectively reduce the scattering neutron background produced by surrounding materials and detectors, and restrain the shape of neutron beam, neutron shield with boron-containing polyethylene (5% of B₄C) coated with 5 cm lead and collimator of parallel hole are designed by MCNP program. The design makes spot of neutron beam flat and uniform, the diameter is about 2 cm, the flux of neutron outside the beam spot is reduced by 5 orders of magnitude, and the flux of gamma is reduced by 3 orders of magnitude. At the same time, neutron absorber (the outer radius is 10 cm, the thickness is 7 cm) is designed, which is used to absorb the scattered neutrons produced by the sample to be measured. The simulation results of MCNP and GEANT4 show that the boron containing polyethylene (¹⁰B₄C mass fraction is 10%) is selected as the processing material of neutron absorber, in which the rate of neutron absorption reaches 80%, and the threshold of sum energy is set to 1 MeV, which meets the requirement of on-line experiment about the measurement of neutron capture cross section.