Based on the observation data of mobile geomagnetic survey points in some areas of Qinghai and Gansu from 2020 to 2021, the abnormal changes of lithospheric magnetic field in the study area were obtained, and the anomalies characteristics of lithospheric magnetic field before the Menyuan, Qinghai M_S6.9 earthquake on January 8, 2022 were systematically analyzed. The results show that: (1) Before the Menyuan earthquake, each component of the lithospheric magnetic field in the epicentral area showed obvious anomalies. The horizontal vector of the lithospheric magnetic field showed abnormal change of direction turning and amplitude weakening near the epicenter; the vertical vector direction showed obvious anomaly of south-north offset. In addition, the magnetic declination had a zero-variation line near the epicenter, showing a high gradient belt distribution. The total intensity and vertical component had a zero-variation line distribution and bending near the epicenter. (2) According to the piezomagnetic effect, the lithosphere magnetic field in the north of Lenglongling fault increased, and the stress mainly released at a low level. The lithosphere magnetic field in the south of Lenglongling fault decreased, and the stress mainly accumulated at a high level. Before the Menyuan earthquake, the piezomagnetic effect caused by the tectonic stress adjustment of Lenglongling fault might be the main reason for the abnormal variation of lithosphere magnetic field in the epicentral region.