Abstract: Objective: To explore the effect of fucoidan on esophageal cancer and analyze its mechanism. Methods: MTT assay was used to analyze the inhibition rate of cell proliferation, Hoechst33258 staining and flow cytometry were used to detect cell apoptosis, and DCFH-DA probe was used to detect ROS level, and Western blot was used to analyze levels of Nrf2, HO-1, NQO-1, Bcl-2, Bax and caspase-3, which were used to observe its effects on fucoidan-regulated cell proliferation and Nrf2/ROS signaling pathway. The tumor formation experiment in nude mice verified the effects of fucoidan on tumor weight, tumor volume and levels of Nrf2, HO-1 and NQO-1 in nude mice. Results: The proliferation of ECA109 cells was significantly inhibited by fucoidan from 1 to 16 µg/mL, and the IC₅₀ was 3.26 µg/mL at 48 h. Compared with the control group (0.1%DMSO), ECA109 cells treated with 1, 2, and 4 µg/mL fucoidan showed obvious apoptotic characteristics, such as nuclear condensation, irregular chromatin contraction and apoptotic bodies, which (extremely) significantly promoted the apoptosis of ECA109 cells and significantly down-regulated the expression level of Bcl-2.The expression levels of Bax and Cleaved-caspase-3 were significantly increased, ROS levels were significantly increased, and Nrf2, HO-1, and NQO-1 protein levels were significantly decreased (P<0.05, P<0.01). Nrf2 overexpression could significantly down-regulate the inhibitory effect of fucoidan on ECA109 cell proliferation, significantly down-regulate ROS levels, and significantly up-regulate Nrf2, HO-1, NQO-1 protein levels (P<0.05). In vivo experiments showed that 50 mg/kg and 100 mg/kg of fucoidan significantly inhibited tumor volume and mass, and down-regulated the levels of Nrf2, HO-1 and NQO-1 in tumors (P<0.05). Conclusion: Fucoidan inhibits the proliferation of ECA109 cells and has significant anti-tumor effect on transplanted tumor in vivo, and its mechanism is related to the regulation of Nrf2/ROS signal pathway.