The largha seal (Phoca largha) is critically endangered in China because of human harassment and habitat destruction. The Bohai Sea is their southernmost breeding site in the world and the only site in China. Long-term studies of the largha seal population in the Bohai Sea were reviewed. The seal population size significantly decreased by more than 80%, from approximately 8000 individuals in the 1940s to 1500 in the 2000s. In the past few decades, the ice formation conditions have been favorable for seals; however, seawater has been severely polluted, and available fish food resources have significantly decreased. In addition, heavy marine transportation has caused increasing levels of seawater and noise pollution. The deteriorating marine environmental conditions in the Bohai Sea most likely affected the largha seal population dynamics, and despite the seal hunting prohibition enacted in 1988, the population has been declining. No successful law has been executed so far by the Chinese government to reduce marine pollution in the Bohai Sea, which may negatively impact seal survival. More efficient protection measures for the species and control measures to improve the marine environment in the Bohai Sea are urgently needed.