On August 8, 2017, an M_S7.0 earthquake occurred in Jiuzhaigou County, Sichuan Province. The Chengdu seismic station is 255km away from the epicentre, and there were tendency anomalies of the amplitude ratios of geomagnetic harmonic wave of the station before the M_S7.0 earthquake. Based on the 1-second sampling data derived from the GM4 magnetometer of the Chengdu seismic station, the temporal variation of the geomagnetic harmonic wave amplitude ratios were analyzed, and the results show that the anomalous characteristics had a down-turning-recovery tendency before the M_S7.0 earthquake, and there was asynchronization between long-period and short-period, and also between north-south direction and west-east direction. The similar variation appeared for the amplitude ratios of the Chengdu seismic station before the M_S7.0 Lushan earthquake.