In this paper the ″Principle of Converted Engagement Reappearance″ is discussed.（1） The realization of generating manufacturing flexible gear under the deformation condition which is equal to that of flexible gear engaged with rigid gear without load;（2） When the wave generator is fixed in the manufacturing system

there is a corresponding relationship between the gear-cuttur and rigid gear; （3） The parameters of harmonic drive gear pair are designed and calculated under the condition of virtual flexible gear: The reference circle of virtual flexible gear is defined as the middle circle of flexible gear

and the pitch circles of harmonic drive gear pair is considered to be equal to this reference circle.The harmonic drive gear pair which are manufactured on the principle of ″Converted Engagement Reappearance″ can conform to the Willis theorem.The manufacturing system based on this principle is efficient and of simple structure.