In this article, the attenuation of broad beam of γ-ray passing through a medium has been researched with Monte-Carlo method and it was pointed out that the beam obeys the exponential rule if an equivalent absorption coefficient is used. A new idea of design forγ-ray thicknessmeter was then advanced for weakening radiation intensity, increasing count, and improving measurement precision. An absorption experiment of broad beam of γ-ray with energy of 60 keV in aluminium has been done and the result agrees with Monte-Carlo calculation.