Yuncheng Basin, the southernmost basin in the Shanxi Graben System, is considered to be one of the earliest formed in this region. However, due to the insufficient study on the strata and the lack of drilling data, the initial time of the Yuncheng Basin has not been accurately constrained, which heavily restricts our understanding about the formation and evolution of the Shanxi Graben System. In this study, borehole SG-1, which penetrated through the entire Late Cenozoic sediments and reached the basement bedrock, was obtained in the northern part of the Yuncheng Basin, and a detailed magnetostratigraphic study has been carried out on the samples selected from the borehole SG-1. Results show that the borehole SG-1 records 17 normal (N1-N17) and 16 reversed (R1-R16) polarity zones which can correspond to the chron C1n-C4An.1n of the global geomagnetic polarity timescale (GPTS). The bottom of the borehole SG-1 is within chron C4An, and the age of the bottom can be extrapolated to 9.1 Ma according to the sediment accumulation rates calculated by two lowermost major polarity transition boundaries. This age provides precise limitation for the initiation of the Yuncheng Basin and the formation of the Shanxi Graben System. The sediment accumulation rates of the Borehole SG-1 was calculated by major polarity boundaries of chrons and subchrons in the borehole SG-1 and showed significant changes at 9.1, 3.6, and 1.2 Ma. Unlike other basins around the Ordos Block which formed much earlier under the influence of western subduction of the Pacific Ocean, the initiation and evolution of the Shanxi Graben system were mainly controlled by the uplift and outward expansion of the Tibetan Plateau after the Late Miocene. The initiation of the Shanxi Graben System indicates that the far-field effect of the uplift and outward expansion of the Tibetan Plateau reached the central North China Craton by at the latest 9.1 Ma. The effect posed a persistent impact on the evolution of the Yuncheng Basin since 9.1 Ma and led to significant changes of the sediment accumulation rates and sedimentary facies at 9.1, 3.6, and 1.2 Ma in the Yuncheng Basin which correspond well to the prologue of the Qingzang Movement, phase A of the Qingzang Movement, and Kunhuang Movement.