Based on self-phase modulation, an analytical model of Yb-doped fiber laser describing its output spectral characteristics has been developed. The output spectral shape, the variation trend of output spectrum along with the increased power, and its characteristics in low-power have been analyzed by experimental measurement. As the experimental results show, this analytical model agrees well with the experimental results, and through this model, the variation of the output spectral line width with the variational output power and the spectral characteristics in low power can be accurately obtained.