In dynamic digital holography, due to different objects at different distances recorded, and constantly changing, the reconstructed image was not always in a clear state. A continuous dynamic observation method based on digital in-line holographic video was proposed. The dynamic tracking focus and video processing algorithms were studied. The center of the measured object in the reconstructed image was used as the center of focusing window in the reconstructing for next frame of holographic video. The focusing window always follows the moving target all the time. Thus ensuring each frame of reconstructed video was in focused accurately, so that the observed target in the video was always in a clear state. In order to improve the processing speed of digital holographic video, the focused distance on a frame was used as the search center of reconstructed distance to avoid repeated operation when dealing with different holograms. Experimental results show that the proposed digital holographic video imaging technology can achieve the moving objects in three-dimensional space of continuous dynamic observation.