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.