A key technology used in the processing of crystal window elements is illustrated by a germanium window element, whose diameter is 20mm and parallellism is 10″. Such a high-quality optical element is difficult to process because the element diameter is small, the parallellism requirement is high and the material is single crystal. Regardless of the conventional idea that crystal element can not be cemented, an‘optical cement’method was used and a set of control measures for high-precision crystal elements processing was summarized. With the method and the control measures, the crystal window element with high quality was made. It was proved that the method can be used in mass production.