Pulmonary fibrosis is a chronic, progressive and fatal disease of the interstitial lung, there still lack of efficient therapy to reverse the prognosis of patients currently. In this study, a fucoidan from Costaria costata was isolated and its anti-idiopathic fibrosis activity was investigated both in vitro and in vivo. The chemical composition analysis results show that Costaria costata polysaccharide (CCP) is consisting of galactose and fucose as the main monosaccharides with a sulfate group content of 18.54%. The results found that CCP could resist TGF-β1-induced epithelial-mesenchymal transition (EMT) in A549 cells by inhibiting the TGF-β/Smad and PI3K/AKT/mTOR signaling pathways. In vivo data found that CCP treatment alleviated bleomycin (BLM)-stimulated fibrosis and inflammation in mice lung tissue. This study suggests that CCP could protect the lung from fibrosis by relieving the EMT process and inflammation in lung cells.