The phenomenon that some extra fine slim stripes overlays on all of the Fraunhofe diffraction bright spots by filament has potential application in untouched optical measurement, manufacturing technique of diffraction grating, encrypted communication and antistealth technology etc. Based on physicl optics diffraction theory, Fourier analysis was applied to sudy the fine stripes and a model was establish for the fine structure of the filament diffraction. The mathematical formula of the diffraction light intensity distribution is given by *. Excluding coefficient errors, the number of the fine slim stripes in the zero-order diffraction spot is direct proportion *. The theory could better explain all phenomena obtained in the experiments.(* Indicates a formula, please see the full text)