Abstract: For validating the calculation of neutron spectrum, which is a key parameter for reactor, the neutron spectrum is usually measured with threshold detector activation method. For resolving the spectrum unfolding issues caused by less number of activation detectors than neutron groups and getting a satisfactory accuracy, the generalized least squares method was adopted and the neutron spectrum unfolding code NSAGLS was developed. This code was verified by benchmark example from IAEA. The results show that the correct neutron spectrum could be obtained using NSAGLS. The code could be used for neutron spectrum unfolding, and it could take into account the uncertainty information of input spectrum, cross section and activity of activation foil.