The Alxa Block is located between the Central Asian Orogenic Belt to the north and the Qilian Orogenic Belt to the southwest. However, the Early Paleozoic tectonic setting of its southwestern margin has long been debated. Researchers who studied the Early Paleozoic plutons in the eastern Beidashan believe that the subduction of Paleo- AsianOcean had reached the southwestern Alxa Block. Others have proposed the post- collision extensional setting for this area based on the Early Paleozoic magmatic rocks in the eastern Longshoushan. It is noteworthy that there is no large fault between the eastern Beidashan and Longshoushan, and the distances between the western Longshoushan and the two orogenic belts are the same as those for the eastern Beidashan and Longshoushan, indicating same tectonic setting. In order to discuss the Early Paleozoic tectonic setting of the western Longshoushan, the Dafosi alkali- feldspar granite, the Wuba biotite granite and the Zhangjiayao granodiorite were sampled for LA- ICP- MS zircon U- Pb dating and element and isotope geochemical analyses. The results indicate that the Dafosi alkali- feldspar granite was formed during Late Silusian (426±2 Ma), and its major elements are characterized by high SiO2 (78. 03%) and total alkali (7. 96%), low CaO (0. 60%) and MgO (0. 13%), with prominent V- type rare earth element pattern and negative Ba, Sr, and Eu anomalies, belonging to aluminous A2- type granite. The Wuba biotite granite and the Zhangjiayao granodiorite were formed during the Late Silurian (421±2 Ma) and the Early Devonian (406±4 Ma), respectively, and both are high K calc- alkaline I- type granites. They have enriched LREEs (La/YbN=10. 69~13. 79), depleted Eu (δEu=0. 56~0. 77), and simultaneously exhibit apparently negative Nb- Ta anomalies and positive Zr- Hf anomalies.Moreover, all the three plutons have enriched Sr- Nd isotopic features and Mesoproterozoic two- stage Nd model ages. Therefore, they may have originated from the partial melting of Mesoproterozoic crust and indicate post- collisional tectonic setting. According to the statistical analysis and regional comparison, the tectonic evolution of southwestern Alxa Block could be divided into the subduction stage of Middle Ordovician- Early Silurian (ca.461~441 Ma) and the post- collision stage of Middle Silurian- Early Devonian (ca. 432~397 Ma). This process is comparable with the evolution of Qilian Orogenic Belt indicating a genetic relationship.