The effects of secondary neutrons on the image quality is simulated using FNRSC (fast neutron radiography simulation code) for a series of scintillator thickness values ranging from 5 mm to 300 mm. The calculated results show that with an incident neutron energy of 14 MeV, the effects of secondary neutrons on the image quality strongly depends on the thickness d of the scintillator for d smaller than 50 mm. When d is higher than 50 mm, this effect is almost saturated. By comparing the image contrast calculated by MCNP (Monte Carlo neutron and photo transport code) with that by FNRSC, we find that the main reasons for the discrepancy is that, the distribution of secondary neutrons strongly depends on that of incident neutrons in the scintillator; contributions of secondany neutrons resulting from energy deposit and fluorescence photons to fast neutron images are different. The results also indicate that a drop occurs in the contrast of neutron images when the elastic scattering cross section between incident neutrons and carbon nucleus is larger.