Abstract: Taking the geomagnetic data of Gansu and Qinghai provinces in Northwest China as the research object, Using the method of geomagnetic polarization, it is found that before the Alashan earthquake of M5.0 on June 3, 2017, The phenomena of polarization over-threshold appeared at geomagnetic stations near the epicenter. The anomalies are as follows: ① Three high polarization anomalies occurred before the Alashan M5.0 earthquake, presenting the short-term characteristics of the earthquake in time. ② The number of abnormal stations increased gradually with the three anomalies. ③ The amplitude increased gradually. ④ The duration of three anomalies grew longer in turn. ⑤ The spatial distribution of three polarization anomalies enlarged with the approaching of Alashan earthquake.

Key words: Geomagnetic, Polarization method, Superthreshold anomaly, Alashan earthquake