The sedimentary and distribution characteristics of polyhalite in the Luobei Sag of Lop Nur Salt Lake were studied using section identification, XRD, SEM, and elemental geochemistry. The results show that polyhalite in the Luobei Sag mainly develops in the middle-upper part of the salt-bearing series. It occurs as either pure-layered polyhalite rocks or in association with glauberite, halite or blodite. There are two origin of polyhalite: replaced and primary. The former is the main type in the Luobei Sag and formed through evaporation of the Lop Nur sulfate salt-type salt lake during the middle and later stages of evolution, with replacement of glauberite or gypsum. The latter may be formed by the enrichment of potassium-magnesium salts due to the supply of calcium-rich brine. The distribution of polyhalite in the Luobei Sag shows a characteristic of thicker in the south and thinner in the north in the south-north direction, and thicker in the middle and thinner on both sides in the east-west direction. The corresponding KCl content also present a similar pattern with higher values in the southern and middle parts, and lower values in the northern and on both sides. These distribution characteristics are clearly controlled by regional tectonic activity, where tectonic-induced subsidence areas are favorable for polyhalite accumulation and mineralization. The shallow burial and large scale of polyhalite deposits in this area make them a potential reserve of potassium salt resources.