Abstract: To meet the current urgent requirement for some type of laser apparatus in the areas of aviation,spaceflight and national defence,etc,this paper successfully deals with developing optical-axis equations and its parallel degree for this type of laser apparatus with detailed analyzing and reasoning by using spatiality and geometry knowledge.At the same time,it also analyzes the problem of beaming spot getting into and out of incepting view field in detail.Then the formula of maximal distance of beaming spot not getting out of incepting view field is given in this paper.The research is valuable to the actual project design and application.