There are several layers of tuff developed in the Sunjiagou Formation in the northern segment of Helan Mountain, which was produced by volcanic ash-fall deposits. In order to constrain the timing of volcanic events at the Permian-Triassic transition period and the formation time of the Sunjiagou Formation in the Helanshan region, zircon U-Pb dating and Hf isotope studies were conducted on the tuff developed within the Sunjiagou Formation in the area. Zircon U-Pb isotope dating results from (264.5±3.3) Ma to (250.0±2.0) Ma. Combined with paleostratigraphy, the main depositional period of the Sunjiagou Formation is determined to be from the late Middle Permian to the early Early Triassic. Zircon grains (age<300 Ma) have negative ε_Hf(t) values from -4.25 to -17.75 and two-stage model age from 1 392 Ma to 2 148 Ma, suggests that the parental magmas of these tuff were mainly originated from the melting of Meso-proterozoic crust of the eastern part of the Alxa Block of the northern margin of the North China Plate. Based on the regional development characteristics of tuff, it is speculated that the volcanic tuff material of the Sunjiagou Formation in the northern segment of Helan Mountain mainly comes from the Langshan area of Inner Mongolia.