In the measurement of pulsed neutrons in the MeV energy range, plastic scintillators are one of the most widely used materials, and their neutron energy spectrum response is the key data required for pulsed neutron energy spectrum measurement. Base on the time of flight(TOF) method, the neutron energy spectrum response of ST401 plastic scintillator with 5 different thicknesses from 0.5 to 10 mm were measured for the 0.5 to 100 MeV energy range on the white neutron source(WNS) beamline of the China Spallation Neutron Source(CSNS). The effects of in-beam gamma rays, the slow component of scintillators produced by the gamma flash and the pulse width of the neutron source on the measurement of neutron spectrum response were analyzed. Due to the boundary effect of the finite volume of the scintillator, the neutron energy spectrum response curves of ST401 with different thicknesses are approximately logarithmic, and proton escape is the main reason for the deviation of the curve from linearity. The thicker the scintillator, the higher neutron energy that deviates from linearity.