The traditional microscope is not suitable to observe the liver fluke cercariae moves in water, because it needs an optical focusing process. A continuous dynamic observation method for liver fluke cercariae based on digital inline holographic video was proposed. The dynamic tracking focus and video processing algorithms were studied. The center of the liver fluke cercariae in the reconstructed image was used as the center of focusing window in the reconstructing for next frame of holographic video, so the focusing window followed the liver fluke cercariae all the time. Thus ensuring each frame of reconstructed video was in focused accurately, so that the liver fluke cercariae in the video was always in a clear state. The observation of liver fluke cercariae experiment was carried out. The holographic reconstructed video of liver fluke cercariae swiming in the water was obtained. The liver fluke cercariae law of motion was summed up by tracked it’s trajectory in the water. The experimental results show that capture a digital hologram can observed fluke cercariae at different positions in water use the digital holography technology, which makes detection more convenient and efficient, and the proposed digital holographic video imaging technology can achieve the continuous dynamic observation of the liver fluke cercariae or other plankton in water.