Characterization studies of a neutron filter of a combination of D2O, aluminum,bismuth and cadmium were carride out experimentally for a neutron beam at the E-1 beam port of THOR. The volume ratio of D2O to Al was 13.6/86.4. The epithermal neutron flux vs thermal is 74/26 at outlet of the beam port, but the fast neutron flux at the same point is far below the detection limit. The rate of gamma dose equivalent vs neutron flux is 1.13pSv·n^(-1)·cm^2 for skin dose and 0.905 pSv·n^(-1)·cm^2 for depth dose.