The invention relates to the technical field of seawater purification, and particularly discloses a plant fermentation extract for regulating seawater quality and a preparation method thereof. The plant fermentation extract for regulating the seawater quality is prepared by mixing raw materials including daylily root, lophatheri herba, seaweed, kelp, an oyster extract, Chinese caterpillar fungus, chitosan oligosaccharide, hirudin, perilla leaves, sandalwood, shredded coconut stuffing, cuttlebone, liquorice, Chinese toona root-bark and selenium yeast according to a certain weight ratio, fermenting, extracting and pulverizing. The raw materials are easy to obtain, the preparation process is simple, the ecological environment of seawater is effectively adjusted, the dissolved oxygen content in water is increased, activation and growth of normal flora and beneficial algae in a seawater ecological system are promoted, and ecological balance of a seawater body is kept.