SVOM-VT has entered the initial phase development stage, and encircled energy is its key performance index. In the development process, it is necessary to determine the encircled energy of the optical lens stage and the system stage. The image recording of a CCD detector includes two imaging processes: one is the pixel integration imaging process, in which the output signal of each pixel is proportional to the area integration of the incident light intensity on the surface of the pixel; the other is the discrete sampling process, in which the continuous graphical object is sampled discretely at the sampling interval of the center distance of the pixel. Based on the data of SVOM-VT, the effect of CCD under-sampling on the encircled energy of detection camera is characterized by simulation and test. Imaging process of CCD pairs of scattered speckles from the lens is a two-dimensional discrete sampling process, as well as the sampling process of discrete signals. This process will lead to low-frequency noise (under-sampling noise) in the sampling of high-frequency signals by CCD detectors, resulting in spectrum aliasing (low-frequency signal distortion) of image signals. Intuitively, the original image is broadened. When the sampling density is increased, this will not be the case. © 2019 SPIE.