High speed imaging technique is an effective method to test the information about pulsed neutron source. Imaging system is usually composed of a pinhole, a scintillator, an image intensifier and a charge-coupled device (CCD) camera. ST401 plastic scintillator is widely used to convert the neutron image into visible light image since it has features of high conversion efficiency and fast time response. When testing a pulsed neutron source of wide energy spectrum, we should evaluate the light yields of ST401 irradiated by neutrons with different energies and make the CCD camera exposed to the light appropriately. A 0.3 MeV pulsed X-ray source is often used to calibrate the imaging system because of its low cost than the D-T fusion neutron source. In this work, a method of evaluating the relative light yield of ST401 irradiated by 0.1-16 MeV neutron to 0.3 MeV X-ray is proposed.Geant4 Monte Carlo software is used to simulate the transport performances of neutrons and X-rays. The software package can simulate the transport process of photons. But the conversion factor of ray energy deposition into photons is unknown. It is difficult to calculate the number of photons generated in ST401 accurately. In this article, we calculate the relative light yield according to the energy of charged particles produced in ST401. Firstly, all information about the particle type, energy deposition, kinetic energy is monitored on event-by-event basis in GEANT4. Secondly, the complete history of the tracks is then used to calculate the light output from the scintillator according to the neutron response functions. Thirdly, the light output caused by charged particles going out of ST401 is deducted. Ratios of average light yield of 1 mm, 3 mm, 5 mm, 1 cm, 2 cm, 3 cm, 5 cm thick ST401 irradiated by 0.1-16 MeV neutron to 0.3 MeV X-ray are given. To confirm the correctness of the simulated result, validation experiment is carried out on IVA pulsed X-ray source and SGIII pulsed neutron source. The simulated ratio of average light yield of ST401 irradiated by one single 14 MeV neutron to 0.3 MeV X-ray has a discrepancy of less than 10% compared with the measured value. Compared with the results of experiment conducted on a constant current source, the simulated results have a maximum discrepancy of less than 44%. If CCD camera exposure 10%-90% of the full scale, the image will have high contrast and information loss can be avoided. According to the simulated results and the neutron yield, exposure can be easily set to be 60% of the full scale by adjusting the gain of the image intensifier. Assume that the simulated results have a 44% discrepancy, the actual exposure will be in a range of 34%-86% of the full scale. Underexposure and overexposure can be avoided by presetting the imaging system sensitivity appropriately based on the simulated results. It implies that the method proposed is effective in predicting the imaging system response to pulsed neutron with wide energy spectrum.