The objective of this study is to extract crude polysaccharide from Enteromorpha prolifera, investigate its structure and convert it into high-quality liquid oil by deoxy-liquefaction. This liquid oil was compared with that directly obtained from Enteromorpha prolifera raw material via elemental, FTIR and GC-MS analyses. The relative nitrogen content of the crude polysaccharide oil (0.97%) was much lower than that of Enteromorpha prolifera oil (4.85%), which can be attributed to the deproteinization process during the extraction of crude polysaccharide. Meanwhile, the crude polysaccharide oil featured high quality such as low oxygen (4.35%) and high calorific value (44.26 MJ/kg). The deoxy-liquefaction technique may be an effective way to convert crude polysaccharide into high-quality liquid fuel.