A parametrized approach, called the modified flexible tolerance, is used to unfold the reactor neutron flux spectrum based on the reaction rates measured by detectors. For the thermal neutron reactor, six parameters are involved. For the neutron flux spectrum of intermediate and fast reactors, the parameters may be reduced to four. for such method, no initial spectrum is required. Our calculations show that the neutron flux spectra obtained for China's SPR and HWRR and Japan's KUR with measured values of reaction rate fit well with those obtained with NEUSPAC code (SAND-II type). The result is also good for the ORR, the MOL-ΣΣ Facility, the YAYOI and the CFRMF. It can be concluded that this method provides an easy and convenient tool for quantitatively understanding the macroscopic characteristics of the neutron flux spectrum.