High-precision KDP optical element is a key component in ICF optical system. The micro-waveness on the machined KDP surface has a significant impact on its transmittance. The effect of micro-waveness amplitude and period on the transmittance of KDP optical element is discussed by Fourier modal theory. Results indicate that:the transmittance increases linearly as the micro-waveness amplitude grows (about 0.5% per 10 nm) when the micro-waveness amplitude is less than 100 nm;the transmittance varies around the central value while the transmittance amplitude remains unchanged,and both of them increase as the micro-waveness amplitude grows. The transmittance is very low when micro-waveness period is between 10.5 μm and 12 μm,and measures should be taken to avoid micro-waveness period in such range. Processing,detection experiments of morphological surface and transmittance of KDP crystal are carried out,and the experimental results are consistent with the theoretical calculations basically.