Abstract: The spectrum of spallation neutron source is of great influence on the multiplication factor and transmutation rate of ADS. It is shown that the spectra of spallation target neutron and fission neutron are similar in MeV region. In this paper, the leakage neutron spectrum and neutron fluence rate of the critical assembly were measured by activation method with In, Al, Mg, Ti, Au, Zn, Ni, Rh, Fe and Co foils as activation foils. The results show that the activation method can be used to measure the neutron spectrum in the spallation target neutron field. The neutron fluence should be up to 5×10¹⁴ cm^-2 in 5 h. The activation foils should be taken out in 1 h after irradiation. The measure order is according to the length of the half-life.