Potassium　dihydrogen　phosphate　crystals　exhibit　excellent　nonlinear　optical　properties　that　are　significantly　affected　by　the　surface　morphology　of　the　crystal.　To　comprehensively　examine　and　characterize　the　morphological　features　of　these　crystals,　the　machined　surfaces　of　workpieces　are　analyzed　using　wavelet,　fractal,　and　power　spectral　density　(PSD)　methods.　First,　the　fractal　method　is　employed　to　analyze　the　features　of　the　machined　surfaces　of　different　materials　and　examine　the　relationship　between　the　surface　roughness　and　fractal　dimension　of　different　materials.　Then,　the　morphological　anisotropy　of　the　machined　surfaces　is　analyzed　using　the　two-dimensional　PSD　method.　Based　on　the　orientation　of　the　machined　surfaces　of　the　workpieces,　the　tangential　waves　on　the　surfaces　are　analyzed　using　wavelet-transform　and　PSD　methods.　From　a　frequency-domain　perspective,　the　scales　of　various　influencing　factor　signals　are　identified.　Additionally,　the　frequency　range　of　the　spindle　vibration　is　determined　based　on　the　machining　experiment.　On　this　basis,　the　cause　of　the　machined　surface　waviness　errors　is　revealed.