Abstract: As the largest Phanerozoic accretionary orogenic belt in the world, the Central Asian Orogenic Belt (CAOB) records the whole process of subduction, accretion and closure of the Paleo-Asian Ocean (PAO). The southern Mongolia, in the central segment of the southern CAOB, has successively developed rock associations of arc-magmatic activity and later extensional activity during the late Paleozoic, which is a key area for studying the accretion and transformation of continental crust. Combined with regional geological characteristics and previous studies, this paper systematically sorted out and summarized the rock association of the key period of Carboniferous-Permian in Southern Mongolia and the magmatic activity can be roughly divided into three stages: Early Carboniferous (350～325 Ma), Late Carboniferous (320～305 Ma), and the early Permian (300～280 Ma). During Early Carboniferous (350～325 Ma), the Southern Mongolia developed typical arc-type magmatic rocks having a southward migration trend. Whole-rock Nd and zircon Hf isotopes show that these rocks have significant mantle contribution. Combined with previous works, this resulted from slab retreating of a series of secondary back-are oceans of the PAO on the north of its main ocean basin. During Late Carboniferous (320～305 Ma), The southern Mongolia was dominated by high silica granites, especially the alkaline feldspar granites and syenites (315～310 Ma), which were produced by the remelting of earlier arc crusts, indicating the cessation of significant consumption of subducted oceanic plates. During the early Permian (300～280 Ma), extension-related magmatic rocks, such as A-type granite, bi-model volcanic rocks and basic dikes, were developed. The above magmatic activities showed the characteristics of high temperature and significant contribution of mantle materials. Therefore, we support that it was caused by the slab breakoff by high-angle subduction resulted from the aforementioned Carboniferous slab retreating. Previous studies show that there are similar rocks, structure and sedimentary records on both sides of Southern Mongolia. Therefore, we propose that the Devonian-Permian subduction and slab retreating of the main basin of the PAO caused the opening, subduction and closure of a series of secondary back-arc basins on the northern side of the PAO, which were accompanied by the last large-scale lateral crustal accretion and its cessation with subsequent slab-breakoff-induced vertical crustal accretion.