The geometry parameters of optical fiber affect the optical transmission and mechanical properties, which are the important indexes to measure the quality of fiber. Near-field light distribution method is recommended in GB15972.20-2008 for the measurement of geometry parameters. In order to distinguish the boundary between fiber core and cladding, the method needs to illuminate the fiber. The end face of the fiber core is a bright spot with unclear edge, so the true edge of the core and cladding cannot be accurately judged. In this paper, the distribution of mode field in optical fiber is analyzed. Theoretically, the solution of electromagnetic vector of mode field satisfies Bessel function, but Gaussian function can also be used under approximate conditions. Therefore, Gaussian function is used to fit the distribution of the fiber core in this paper, and the real edge of the fiber core and cladding can be obtained from the Gaussian function after fitting. This method is a further improvement on the measurement method of GB15972.20-2008. The experimental results show that when the cutting effect of the fiber is not good or the imaging quality is poor, the Gaussian function method fitting with mode distribution can still ensure the repeatability of the measurement and the stability of the measured data.