Based on six consecutive periods of mobile geomagnetic observation data in Tangshan and its surrounding areas from April 2018 to September 2020,the spatial and temporal variation data of the lithospheric magnetic field on a half-year scale were obtained through data processing. In conjunction with the Fengnan M_S4.5 earthquake on December 5,2019,the spatial and temporal evolution characteristics of the lithospheric horizontal vector near the epicenter were systematically analyzed. The results indicate that: ①In the dynamic evolution process of the lithospheric magnetic field's horizontal vector,an anomaly in the horizontal vector appeared in the epicenter area of the Fengnan earthquake one year before the event. Over time,the weak variation area of the vertical magnetic field exhibited an east-to-west spatial evolution. ②The magnitude of the horizontal vector in the epicenter area of the Fengnan earthquake decreased initially and then increased before and after the earthquake. The horizontal vector magnitude was smallest six months before the earthquake,with the Fengnan earthquake occurring during the reverse increase of the horizontal vector modulus. ③According to the relationship between the sub-instability theory and the horizontal vector variation of the lithospheric magnetic field,identifying seismomagnetic precursory anomalies may involve searching for potential anomaly areas where the horizontal vector of the lithospheric magnetic field shows directional and weak variations over multiple periods. This should be combined with the dynamic variation of the average value of the horizontal vector modulus in the anomaly area. These findings suggest a method for identifying seismomagnetic precursor information before an earthquake by monitoring specific variations in the horizontal vector of the lithospheric magnetic field.