On November 23, 2017, M_S5.0 earthquake occurred in Wulong, Chongqing. In this paper, we calculated the three component data of the geomagnetic station by using the harmonic amplitude ratio method, and obtained the ratio of 10~60min period harmonic amplitude, and then discussed the variation characteristics of the deep resistivity before the earthquake. The results show that the distance of anomalies of geomagnetic harmonic amplitude ratio before wulong M_S5.0 earthquake is less than 280km, where the stations with anomalies are concentrated in space and distributed on the side of the epicenter of the earthquake. On the projection of the source mechanism solutions, the anomalies are all located in the region under the initial p-wave movement, with a quadrant distribution feature. The Y_ZHy harmonic amplitude ratio of the stations with anomalies are characterized of decreasing variation similar to the anomalous resistivity. In the process of descending change, the harmonic amplitude ratio of Y_ZHx and Y_ZHy show different turning points. Y_ZHx experienced two asynchronous transitions, in which the first about 1.8~2 years before the earthquake, and the second about 1 year before the earthquake. Y_ZHy of the stations within 100km from the epicentre of the earthquake appeared the phenomenon of transition time from deep to shallow in the transition process. Y_ZHy anomaly amplitude shows that the anomaly amplitude of 20~60min generally decreases with the increase of the epicenter and increases with the increase of the period. The amplitude of Y_ZHx anomaly does not change with the epicenter or the period.