Characters of f-θ lens system are analyzed based on diffractive collimation technique and a kind of profile measuring method of large aspheric surface is put forward.The diffractive collimation beam produced by two-dimensional phase plate is the measuring base and the dark line of diffractive spot is displacement base.Through combining ultra-precise line motion with ultra-precise rotary motion,the three-dimensional profile of large hither plane aspheric surface is obtained by rotary scanning mea...