In scientific research and engineering application, improving the power of fiber device is an important topic, which leads to observably rise of temperature in fiber core at the same time. In this paper, Thermal effect and its influence on absorption spectrum and lifetime of Erbium-doped fiber are studied with numerical modeling. Lorentz broadening of sub-levels is used to build the mathematical relationship between temperature and absorption spectrum. The McCumber Theory is applied to deduce the lifetime of Erbium-doped fiber in different temperature. Temperature experiments of absorption and emission spectrum from 25 to 900 ℃ are carried out, which show that the wavelength of absorption peak near 980nm increase at rate of 0.625 nm/100 ℃, the ratio of absorption peak near 1 530 nm declines at a rate of 0.001 9 dB·(m℃)-1 and the broadband of absorption spectrum near 1 530 nm increase with rising temperature. The linear variation of lifetime and peak absorption in experiment proves that the theoretical model is reasonable when the temperature is below 600 ℃.