• A heteropolysaccharide Sargassum Zhangii fucoidan (SZF) has an Mw of 111.28 kDa. • The main monosaccharides of SZF include galactose, fucose, xylose, and uronic acids. • SZF possesses a main chain of (1,4)-α-d-linked-galactose, (3,4)-α-l-fucose, etc. • SZF has significant immune-stimulating properties compared to commercial fucoidans. • SZF has great potential to act as ingredient of functional food or immunodulator.