无论是通讯光纤还是医用光纤，光纤的几何参数总是评价其质量的重要指标。灰度法是国标 GB15972.20-2008中的建议方法，但该方法在拟合过程中会出现拟合圆与椭圆的中心不重合，存在测量原理上的缺陷。且当光纤的切割效果与照明条件发生改变时，往往导致测量数据的不稳定并带来误差。本文用更符合光纤端面实际的任意椭圆函数 (非标准椭圆)，且仅用这一种函数拟合的方法求取光纤几何参数，从而从根本上消除由圆拟合与椭圆拟合的中心不一致带来的原理缺陷。同时，由于在计算各个参数时不需要图像分布灰度的具体值，从而降低了对测量条件的要求。实验表明，本文方法能有效提高仪器测量结果的稳定性和一致性。

The fiber geometry of communication fibers and medical fibers are always standards to evaluate the quality of optical fibers. The measurement of fiber geometry with gray scale method is one of the commonly used measurement methods. It is also the proposed method in the national standard GB15972.20-2008. In this method, the fiber geometry is obtained by fitting the elliptical curve and fitting the circular curve in two steps, but the center of the two curves may not be coincided. Thus, there is a defect in the measurement principle in the method. The measurement of fiber geometry with gray scale method has a high requirement for cutting effects and lighting conditions. When measurement conditions change, it often leads to the instability of the measured data and brings errors. In this paper, we use the arbitrary elliptical function (non-standard ellipse) which is more suitable for the fiber end face, and only use this function fitting method to get the fiber geometry to fundamentally eliminate the principle defect caused by the inconsistent center fitting between the circle fitting and the ellipse fitting. At the same time, the re-quirement of measurement condition is reduced, because the specific value of image distribution grayscale is not needed when calculating each parameter. Experiments show that this method can effectively improve the stability and consistency of measurement results.