The utility model provides a boracic gas film fast neutron detector which comprises a packaging member having a hollow cavity, a plastic scintillator array which is arranged in the cavity and comprises a plurality of plastic scintillator elements, and a boracic gas which fills and is hermetically sealed in the cavity, wherein there exit a gap between adjacent plastic scintillator elements. The boracic gas is formed into a boracic gas film in the gap between the adjacent plastic scintillator elements. By the adoption of the fast neutron detector provided by the utility model, scarce and expensive 3He gas is not needed at all, a redundant boron coating process is not necessary, and reliability which a signal accords with is raised. The boracic gas film fast neutron detector is suitable for measurement of environmental background neutrons and can be widely applied in detection of radioactive substances at places such as ports, harbors and the like.