The mode field diameter is an important parameter of single-mode fiber, and the GB.15972.45-2008 recommends using the far-field variable aperture method to measure it. This paper analyzes the distribution of the propagating light field in a single-mode fiber. The mode behavior of the light field is the solution of the Helmholtz equation, which in theory should satisfy the Bessel function. In this regard, a method using Bessel function to fit the optical field distribution of the fiber based on the far-field variable aperture method is proposed, and the mode field diameter is calculated from the fitted mode field distribution curve. Compared with the commonly used far-field variable aperture method, when the measurement data is normal, this method has the same measurement accuracy. When there are errors in the measurement data, this method can still ensure the stability and accuracy of the measurement results.