Because KDP crystal is a soft material, when it is machined by single point diamond turning, vacuum chuck will result in surface waviness on crystal′s surface. Surface waviness will seriously affect the quality of high power laser beam, or even destroy optical elements, due to the nonlinear effect of high power laser. In order to improve the ultra-precision machining of KDP crystal, power spectral density is used to detect the machined surface of KDP crystal, and to analyze error sources. It is discovered that processing methods should be chosen according to the shape of vacuum chuck, and the use of vacuum chuck should be avoid in the vertical direction.