Due to the frequent occurrence of Enteromorpha prolifera "green tide" in recent years, how to solve the ecological disaster brought by the green tide and make maximal use of the Enteromorpha prolifera have become an urgent problem. In order to realize the resource utilization of marine biomass, we extract nanofibrillated cellulose (NFC) with high aspect ratio from Enteromorpha prolifera. Filamentous NFC is successfully extracted at nanometric scale with a diameters range of 20~40 nm, a length of several microns, and a crystallinity index of 57.2%. Then NFC/polyvinyl alcohol (PVA) composite films were prepared with NFC as fillers. In addition, for comparison, nanocrystals cellulose (NCC) with small aspect ratio was isolated from microcrystalline cellulose (MCC) to prepare NCC/PVA composite films. It is shown that because Enteromorpha proliferata NFC has a higher aspect ratio, it is easier to interpenetrate with PVA to form a dense network, and the lower concentration ensures the better dispersion of NFC in the matrix, so its enhancement effect on PVA composite films is better than that of NCC. When the NFC content was 0.1 wt %, the light transmittance of NFC/PVA composite film could reach more than 94%, and the swelling degree was 250%, which is 29% lower than that of the pure PVA film, the water resistance was improved. At the same time, the tensile strength, elastic modulus and toughness were 99.1%, 1547% and 117.9% respectively, which are higher than those of PVA.