The Er3+ doped (the mass fraction is 1%) Ga5Ge20Sb10S65 chalcogenide glasses are fabricated and the refractive index, absorption spectrum and emission spectra are measured. The spontaneous emission probability, absorption cross-section and emission cross-section are calculated using the theories of Judd-Ofelt and the Futchbauer-Ladenburg equation. Based on the rate equations and light propagation equations, the numerical simulations on mid-infrared gain characteristics for the Er3+ doped Ga5Ge20Sb10S65 chalcogenide glass fiber are calculated. Effects of energy transfer between Er3+ levels due to cross-relaxation, cooperative up-conversion and excited state absorption are considered in simulation. The results show that the Er3+ doped Ga5Ge20Sb10S65 chalcogenide glass fiber has a bigger signal mid-infrared gain and wider mid-infrared gain spectrum, and its maximum signal gain and gain width at 20 dB exceeds 40 dB and 200 nm, respectively. The results indicate that the Er3+ doped Ga5Ge20Sb10S65 chalcogenide glass is a better gain medium which can be applied to broadband amplifiers in mid-infrared.