This paper studies an omni-directional vision system and its imaging model. The omni-directional vision sensor uses a combination of lenses and mirrors placed in a carefully arranged configuration to capture a much wider field of view for the autonomous soccer robot. Mirror parameters of omni-directional vision sensor were designed under the single viewpoint constraint,and dimension of the omni-directional vision system was determined. A set of models including reflect and refraction model were built. The relation between image point and corresponding world point was determined,which offers the essential model and algorithm for omni-directional image process. Their practicability is tested through the simulation experiments.