The Longshoushan Group is a part of Precambrian metamorphic basement of the Alxa block. Its origin is particularly important for constraining the western boundary of the North China Craton(NCC)and understanding the evolutionary history of China's cratons. In this study, LA-ICP-MS U-Pb dating technique was used to determine the ages of different rocks from Longshoushan Group and Dunzigou Group. The analytical results of detrital zircon grains from the metasedimentary rock in the Longshoushan Group gave two age populations: ～2.01 Ga and～2.15 Ga. The magmatic zircons and metamorphic zircons from the granitic gneiss yield age spans of 2.04～2.17 Ga and 1.89～1.93 Ga, respectively. These ages indicate that the Longshoushan Group was formed in Paleoproterozoic. The metamorphic zircons from amphibolite in the Longshoushan Group yielded a metamorphic age of ca.1.85 Ga, indicating that the Longshoushan Group underwent an amphibolite-facies metamorphism during late Paleoproterozoic. Detrital zircon ages of the metasedimentary rock from the overlying Dunzigou Group vary from 2.03 to 2.05 Ga, suggesting a single Paleoproterozoic provenance. These data imply that the southern Alxa block had an affinity with the NCC during early Precambrian period.