The invention discloses a method for improving the radiation resistance of an erbium-doped optical fiber amplifier. When the ratio of equivalent erbium doping radius of an optical fiber to the core diameter a of the optical fiber is constant, the core diameter a is increased to improve the radiation resistance of the erbium-doped optical fiber amplifier, and the formula of a is as shown in the description, wherein lambda c is the cut-off wavelength of the optical fiber, nco is the core layer refractive index of the optical fiber, and ncl is the cladding refractive index of the optical fiber. In the situation that the ratio of the erbium doping radius of the optical fiber to the core diameter is set, the radiation resistance can be remarkably improved through increasing the core diameter.