This paper discusses the thermal effects of optical components and media gas in inner propagation of high energy laser system, which is very important for the performance. The thermal aberration models of optical components and media gas, the influencing factors, and the changing laws are introduced. For optical components, some physical factors are emphatically analyzed, including absorptivity, characters of the materials, and laser spot distributions. The results show that the level of thermal aberrations depends on the absorptivity, and spatio-temporal characters depends on the material and the distribution of laser spot. For media gas, the variation of thermal aberrations depend on switch of physical mechanism, heat conduction and heat convection for temperature increment, and the law of time dependent thermal aberrations are studied carefully. Methods and measures of reducing thermal effect are introduced for closed and opened system. The multi-physical module of a software named Easylaser is introduced and is used to simulate the thermal effects of a laser inner propagation, including reflective mirrors, a optical window, a spectroscope, and media gas. The results show that the aberrations between reflective mirrors and the optical window, and between gas and the optical windows could be complementary. The characters of laser spot in far-field are also simulated. All the results demonstrate that Easylaser can be used to simulate and analyse the thermal effects in high power laser system.