Using the 5SDH-2 Accelerator at the Radiation Metrology Center of CIAE, the experiment of neutron energy response of the thin ST-401 scintillator has been performed. Two T(p,n)3 He and D(d,n)3He reactions are chosen for generating the neutrons. The neutron beam is collimated with the multiplex shielding system. The source intensity is monitored by the proportional counter or semiconductor proton recoil telescope. The neutron response curves of ST-401 scintillators with thickness from 0.16mm to 2.00mm are measured versus neutron energy. The uncertainties of experimental results are discussed in detail. The results show that the sensitivity increases with the scintillator thickness, and for the same thickness, the slope of energy response curve does not change greatly with the increase of neutron energy.