The invention discloses a method for calculating light source energy of an infrared optical system with a scattering element. The application of the current infrared optical system becomes more and more popular; to meet requirements for use, the infrared optical system is generally equipped with a proper light source. If the selected light source energy is too low, the requirements for use can not be met; if the light source energy is too high, resources can be wasted. With regard to the optical system with a scattering element in a light path, a normal method for calculating light source energy of a general optical system is unavailable, which results from that the optical systems in front of and behind the scattering element have different geometric degrees. Therefore, the accurate calculation of the light source energy in the optical system with the scattering element has a high value for project application. The method for calculating light source energy of the infrared optical system with the scattering element, disclosed by the invention, is established according to a derived energy calculation formula by deriving a relationship between the radiance and the radiation existence at the viewing diaphragm in the light path, so that the calculation of the light source energy of the infrared optical system with the scattering element is realized.