We are studying about NCT accelerator-base neutron irradiation field with variable energy spectrum, aiming at the more positive dose-distribution control for wider application to various tumors and various body-parts. In the design study, we adopted the combination of Li-7(p,n)Be-7 reaction and protons of nearly 2.5-MeV energy. The neutron energy spectrum shift is performed by changing the heavy-water thickness. The parametric surveys were performed by two-dimensional transport calculation. From the parametric survey results, it was confirmed that the accelerator-base irradiation field with variable neutron spectrum was feasible for the beam quality. Moreover, it was confirmed that the thermal neutron flux distribution was improved employing the regional-filtering technique.