Abstract: In order to obtain a small angle monoenergetic neutron source, the Monte Carlo software is used to design the shield collimating structure of the small 4 neutron source with energy of 2.45 MeV produced by a D-D neutron tube. The shield collimating structure is divided into a collimator and a capture cave. The collimator is made of three layers of iron, boron containing polyethylene and lead, which is used to shield the stray neutrons from the irradiation outside. The main function of the capture cave is to increase the number of elastic scattering in the opposite direction, thus reducing the proportion of the low energy scattered neutrons. The optimum thickness of the material and the size of the aperture and the optimum structure of the capture cave are obtained by MCNP simulation. Finally it is proved that the neutron flux of neutrons with 2.45 MeV energy in the radiation field is three orders of magnitude higher than that outside the radiation field. The low energy neutron flux in the radiation field is one order lower than the 2.45 MeV neutron flux, the total dose rate of both neutron and gamma outside the walls is below 2.5 Gy/h. This research has practical value for small angle neutron source with single energy, and it has important significance for the study of the verification of validity of neutron dosimeter and performance test of neutron monitor.