Abstract: (n,2n) cross section can be measured through the direct measurement method with a 4 spherical 3He neutron detector. The neutron beam must be collimated in such experiments. A satisfactory design of the collimator is required to: (1) improve the uniformity of neutron flux at sample position and guarantee enough decreasing amplitude of the border of neutron beam at the same time; (2) reduce the scattered neutron background at low energy region. In this work, using FLUKA and MCNPX codes, the comparison between cylindrical collimator, conical collimator, and three double-truncated conical collimators with different slope on the uniformity of neutron flux and the proportion of low energy neutron was studied. The result shows that the double-truncated conical collimator with low slope can satisfy the experiment requirement. At the same time, the response of the detection system was compared after neutrons collimated by five collimators respectively with and without sample. The result shows that when neutrons are collimated by the double-truncated conical collimator with low slope, the detection system has the higher count rate with sample, and the lower background without sample. In addition, the collimation efficiency affected by the slope of collimator outlet was also compared. The double-truncated conical collimator with low slope was finally chosen as the neutron collimator and the materials of the collimator are red copper, stainless steel, polyethylene and lead. The diameter at the beginning of the collimator aperture is 2.64 cm and its length is 137 cm. The beam diameter after collimation at the sample position is 3.2 cm.