This work is aim at finding the reason why the measured n/γ sensitivity ratio of film detectors is lower than the theoretical result.It was found that in theoretical calculation

the influences of air and detector’s window are omitted

but secondary electrons produced from gamma interaction with this material will deposit energy in the film

and this increases the detector’s gamma sensitivity.Through MCNP simulation and measurements in vacuum

it is approved that the impact of secondary electrons is obvious

and the thinner the film

the higher the interference.As air is the main source of secondary electrons that limits the improvement of the film detector’s n/γ sensitivity ratio

vacuum structure is needed for new film detector with high n/γ sensitivity ratio.