A flying spot forming apparatus and a designing method for the flying spot forming apparatus are disclosed. The flying spot forming apparatus comprises a radiation source (P) and a shielding body (100). A side wall of the shielding body (100) is provided with at least two pairs of helical grooves. Each helical groove is provided with a predetermined slope. A first incident groove is adjacent to a second incident groove. A head end of the first incident groove is higher than a head end of the second incident groove. A tail end of the first incident groove is higher than a tail end of the second incident groove. The tail end of the first incident groove and the head end of the second incident groove are spaced with a predetermined distance. The tail end of the first incident groove is not higher than the head end of the second incident groove. A first axial cross section of the shielding body (100) intersects with the tail end of the first incident groove. A second axial cross section intersects with the head end of the second incident groove. An included angle of the first axial cross section and the second axial cross section is greater than 0 degree. The flying spot forming apparatus can improve tensile performance of the shielding body.
