Kjellmaniella crassifolia was cultured firstly in recent years in Dalian, China. To elucidate the hypolipidemic effects and functional mechanisms of fucoidan (KF) in K. crassifolia, hypolipidemic activities were studied using the established hyperlipemia Wistar rat model, corresponding chemical compositions and structures were analyzed. The results show that KF is 71.68% carbohydrate and 20.04% sulfate. The monosaccharides of KF mainly consist of fucose (31.89%) and galactose (23.54%), with sulfate bound at the equatorial C-4 position and at the axial C-2/3 position of fucpyranose. These findings show that an animal’s hyperlipidemia and antioxidative stress can be remarkably improved by KF and that the effects of KF are time- and dose-dependent. The molecular mechanism of KF modulates hyperlipidemia by inhibited cholesterol and aliphatic acid synthesis, accelerating the mitochondrial β-oxidation or peroxisomal oxidation degradation of aliphatic acid in vivo. These findings suggested that fucoidan extracted from K. crassifolia could be developed as a natural food used to treat hyperlipidemia.