Abstract: In order to research on the correlation of gamma energy spectrum and absorbing medium, based on the method of Monte Carlo, MCNP5 was used to simulate the changes of gamma energy spectrum after gamma photons for 1.46 MeV eitted by ⁴⁰K through absorbing mediums on ¹²C,²⁷Al,³²S,⁵⁶Fe and ⁶⁵Zn, and summarizes the relationship between the characteristics of gamma energy spectrum and the atomic number, the density and thickness of the materials. The results showed that the atomic number of absorbing medium has great influence on low-energy parts of gamma energy spectrum. With increasing atomic number, low-energy peak height gradually decrease, and peak position moves to higher-energy direction. Both medium density and thickness didn’t change peak position of gamma energy spectrum except peak height and Compton plateau. Peak height and Compton plateau of gamma energy spectrum decrease gradually with increasing medium density. When the thickness of absorbing medium increased, low-energy peak height and Compton plateau gradually were changed to higher.