在天文光纤光谱仪中，阶跃型圆柱多模光纤的出射场在有些情况下会变成环形分布的光斑而不是具有高斯分布的圆形光斑，这将对天文光谱测量造成重要影响，不仅引起伪光谱漂移导致光谱测量精度降低，而且还产生伪光谱结构使得原本复杂的天文光谱更加难以分析。利用光线理论对阶跃型圆柱多模光纤的光学传播性能进行分析，揭示了造成环形出射场形成的两大因素：一是入射偏移（包括中心偏移和角度偏移），另一个是圆柱光纤旋转不变性导致的空间光线传播方式。并通过建立入射中心偏移量与出射场环形分布的数学模型，研究了随着中心偏移量增大环形出射场的形成和演化过程，理论分析与实验现象吻合。

In astronomical fiber spectrograph, the output field of step-index cylindrical multimode fiber under certain circumstances can become a ring profile rather than a spot with Gauss distribution. The change can greatly affect the measurement of astronomical spectroscopy, deteriorate the precision of spectrograph, and make the astronomical spectrum too complicated to analyze because of pseudo drifts and structures in spectrum. The propagation characteristics of step-index cylindrical multimode fiber are analyzed by ray optics, and thus the two major factors causing the formation of the ring output field are revealed. One is the coupling deviation, including center deviation and angle deviation. The other is the trace of the ray due to the rotation invariance of cylindrical fiber. A mathematic model is established to study the dependence of the distribution of output field on the center deviation. The formation and evolution process of the ring output field as the center deviation increases are studied, and the results show that they are in accordance with experimental results.