Holothurian glycosaminoglycan (hGAG) is extracted from the body wall of the sea cucumber, and previous studies have shown many unique bioactivities of hGAG, including antitumor, anti-angiogenesis, anti coagulation, anti thrombosis, anti-inflammation, antidiabetic effect, antivirus, and immune regulation. The effects of 3W and 5W molecular weights hGAG with hematoporphyrin derivative-photodynamic therapy (HPD-PDT) on lung cancer were investigated. Human lung adenocarcinoma A549 cells were divided into 6 groups: control group, 3W molecular weight hGAG group, 5W molecular weight hGAG group, HPD-PDT group, 3W molecular weight hGAG + HPD-PDT group, and 5W molecular weight hGAG + HPD-PDT group. Cell morphology was observed under inverted phase contrast microscope. Cell proliferative activity was detected by CCK8 and cell apoptosis was assayed by Hoechst33258 staining and flow cytometry. The results showed that two different molecular weights hGAG could inhibit proliferation, promote apoptosis rates of A549 cells, and enhance the sensitivity of A549 cells to HPD-PDT. The combined use of hGAG and HPD-PDT has synergistic inhibitory effects on A549 cells, and the effects of 3W molecular weight hGAG are better than that of 5W molecular weight hGAG.