An improved model of erbium–ytterbium codoped fiber amplifier (EYDFA) considering the effects of radiation is proposed. In this model, terms of radiation-induced losses are introduced into power propagation equations, combined with the simplified rate equations to simulate radiation effects on EYDFA performances related to space missions. A method for recovering background losses and radiation-induced losses of EYDFA is illustrated. Numerical simulation tools are based on a homemade particle swarm optimization procedure. In the experimental aspect, two amplifiers of the same type were irradiated with Co60 gamma rays up to a total dose of 10 and 50 krad, respectively. Compared with the experimental data, the curve of output power drawn by the model is in good agreement with the experimentally measured values, validating the validity of the theoretical model. Using this method that combined experimental testing of online monitoring with theoretical simulation provides a new way to evaluate the performance degradation of commercial amplifiers in a radiation environment.