The invention provides a boracic gas film fast neutron detector. The detector comprises a packaging member, a plastic scintillator array and boracic gas, wherein the packaging member has a hollow cavity, the plastic scintillator array is arranged in the cavity and comprises a plurality of plastic scintillator elements, a gap exists between every two adjacent plastic scintillator elements, the boracic gas is filled and is hermetically sealed in the cavity, and the boracic gas forms a boracic gas film in the gaps between the adjacent plastic scintillator elements. By the adoption of the fast neutron detector provided by the invention, scarce and expensive 3He gas is not needed at all, a redundant boron coating process is unnecessary, and the reliability of signal according is improved. 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 and harbors.