Cathepisn D plays a key role in early process of apoptosis before mitochondrion damage and caspases activations, and also involves in Alzheimer's disease (AD). Glycosaminoglycans (GAGs) have been suggested to inhibit the progress of apoptosis. Fucoidan, a nature GAGs mimetic, is shown as a potential candidate for neuroregressive disease. Here we reported PC12 cells response to oxidative stress with clear cathepsin D release, followed by caspase-3 activation. We found that fucoidan treatment can alleviate cathepsin D and caspase-3 activation, and improve cell survival. Furthermore, for the first time, fucoidan was shown to directly inhibit human liver cathepsin D by a dose-dependent way. These results support that cathepsin D involves in early apoptosis, suggest that fucoidan can decrease apoptosis at lysosome-cathepsin D level, which opens a new therapeutic approach to AD.