Laser diffraction (LD) has many obvious advantages for measuring. However, the measurement accuracy is limited by a number of factors, such as imaging noise, sensor threshold, and fitting methods. In this paper, we present a novel method for measuring filament diameter based on image-based fitting, which maintains more information. Before fitting the diffraction image, image processing is applied to solve the problem of image noise and the non-linear response of the charge-coupled device (CCD). Then, a fitting formula is established based on the distribution of laser intensity on a diffraction image, and the fitted results are solved with the Levenberg–Marquardt (LM) algorithm. Finally, the initial parameters of a fit are obtained by calculation, which speeds up the calculation and improves the accuracy of the fitting. The measurement accuracy of this method is verified by experimental and theoretical analysis. In experiments, the filament diameters of 125 and 125.2 μm are measured with a relative error of approximately 0.12%, Furthermore, the superiority of this method is demonstrated by comparing the measurements with other methods. To verify the stability of the measurements, filament diameters of 110–180 μm are chosen to be measured with a relative standard deviation of less than 0.14%.
