In femtosecond laser micromachining lateral distribution of the focus light spot directly affects laser machining accuracy, resolution and surface roughness. Focused spot was modulated by using laser beam spatial shaping technology, lateral ultra diffraction theory was studied and analyzed, One kind of tetracyclic lateral modulation phase plate was designed by using optimization algorithms. The radiuses of the phase plane are r₁=0.16、r₂=0.27、r₃=0.49, the peak energy S is 0.38, the focal spot size GT is 0.74, sidelobe energy MT is 0.20. The experiments were processed using a photochromic material before and after adding the phase plate in a laser micromachining system, the experiments results show that the performance of this super-diffraction phase element is particularly desirable in the femtosecond laser micromachining system, which can reduce the size of the point.