Abstract: Chlorosulfonated polyethylene (CSM) is a widely used special rubber with excellent properties. There is still a wide gap in quality between the domestic and foreign products. In particular, the high performance CSM products are lacked in our country. In this paper, the difference in the molecular chain structure between domestic and foreign CSM with the same grade was investigated. The results indicate that the chlorine atoms distribution along the molecular chain of CSM product abroad is more homogeneous, leading to the lower glass transition temperature and crystallinity, and thus the stronger tensile strength and better thermal stability and cold resistance compared to the domestic one.