The invention relates to an alginate-based hydrogel dressing and a preparation method thereof, and belongs to the technical field of medical supplies. Raw materials of the invention are a compound of sodium alginate or other alginates and a thickener, a moisturizer and other functional matters, are subjected to swelling dosing, pre-crosslinking, film coating, spray crosslinking, constant-temperature crosslinking plasticizing and film-covering shear forming, and are then prepared into a three-dimensional water-containing grid structure through covalent cross-linking and ion cross-linking. The medical hydrogel dressing is a topical gel dressing, has good moisturizing performance, toughness and adhesion, is mainly suitable for superficial skin damage, such as cut wound, lacerated wound, scratch, burns, suture wounds and the like, protects skin wound healing of a patient, has effects of easing pain, inhibiting bacteria, resisting inflammation, keeping the wound moist, promoting wound healing, and improving the wound repairing quality, is more suitable for treating bedsore wounds, ulcer wounds and other sites hard to be bound after a surgery, can also be used for non-wound sites, and plays a role of moisturizing.