Serious edge effects of potassium dihydrogen phosphate (${{\rm KH}_2}{{\rm PO}_4}$KH2PO4, KDP) manufactured using single-point diamond turning (SPDT) often result in disqualification of the transmittance wavefront for high-power laser systems. In this paper, based on the theoretical analysis of sucker hole configuration and the pressure distribution law under the vacuum chuck condition of crystal elements, the influence of sucker hole configuration on the transmittance wavefront root-mean-square gradient (GRMS) is verified through fly-cutting experiments. By adopting the newly designed vacuum chuck, the vacuum-chucking quality is effectively improved, and the edge effect is accordingly suppressed in the SPDT. Moreover, the accuracy of the transmittance wavefront GRMS has an improvement of about 25% under the same processing parameters.