By taking Sun Synchronous Orbit(SS-O) for example,the relationship between swath width and orbital elements(especially the orbital altitude) was discussed,and a SS-O design method for satisfying the swath width requirement of space borne optical sensor was also given.The theory of non-spherical earth perturbation and the definition of SS-O were discussed.Then,the relationship among nodal period,fundamental interval and orbital altitude was educed by deriving the definitions of fictitious node.Finally,the restriction conditions of orbit altitude satisfying swath width requirement based on the discussion above were given.The definition of fictitious node very helpful to comprehend the theory on SS-O was first put forward.Furthermore,the mathematical relationship between SS-O nodal period and orbital semimajor axis was deduce,which has been available for SS-O design.