Based on the erbium-doped fiber Giles' model, a numerical model of an erbium-doped fiber ring laser is established and stimulation parameters are set according to the actual used σ structure, which are used to simulate the impacts of forward Amplified Spontaneous Emission (ASE), backward ASE and two-way ASE on the performance of laser, and to obtain the optimum parameters of laser. Stimulation results show that when the laser oscillates, the consideration of the direction of ASE has little influence on the output optical power amplitude or Giles gain particles distribution, while the existence of backward ASE results in the cut-off length of erbium-doped fiber. The comparison between the experiment and the simulation indicates that this model can describe the performance of laser precisely, so that it can be used to optimize the actual laser design.