The utility model discloses an online back-flushing ultrasonic seawater filter, which comprises a shell, a filter core and a water-stop sheet, wherein the water-stop sheet is positioned in the shell; the space in the shell is partitioned into a pre-filtering space and a post-filtering space; an opening of the filter core faces downwards, and is embedded on the water-stop sheet; and an ultrasonic vibration plate is arranged in the filter core. The online back-flush ultrasonic seawater filter has a more reasonable design, and is more complete and reliable; and in a working process, marine organisms can be effectively prevented from entering a seawater system, the problems of blocking and pollution discharge existing in the filter core are solved, the regular cleaning frequency is lowered, the problem of corrosion of the filter is solved, the filtering accuracy can be greatly increased, the filtering effect of the seawater filter is enhanced, the workload and economical cost due to frequent cleaning, maintaining and replacing of the seawater filter are reduced, and the service life of the seawater filter is prolonged. The online back-flushing ultrasonic seawater filter can well adapt to a service environment in a use process, has high suitability, and has very important and positive significances.