The invention discloses a leaf fertilizer composition comprising sodium alginate oligosaccharide, which is characterized in that the composition comprises sodium alginate oligosaccharide, amino acid and microelements that can promote plant growth significantly, wherein sodium alginate oligosaccharide is also known as sodium alginate, sea-tangle glue and algin, is natural polysaccharide carbohydrate extracted from sea tangle, and is a copolymer comprising a-L-mannuronic acid (M), b-D-guluronic acid (G) connected with a-L-mannuronic acid (M) by 1,4-glycosidic bond, and different GGGMMM fragments; a molecular formula is (C6H7O6Na)n; the degree of polymerization is 2-10; the uronic acid constitution M/G is equal to 7:3; the uronic acid content is greater than 90%, and is 2-3% of the total content; the content of a microelement leaf fertilizer is 2-20%; and the microelement leaf fertilizer comprises at least two of sulfur, boron, ferrum, manganese, copper, zinc and molybdenum. The composition is mainly applicable to industrial crops, fruit and vegetable crops, grain crops, economic trees, flowers, lawns and the like.