Abstract: Based on the 1 second sampling geomagnetic data derived from the eight stations of geomagnetic array in Yunnan Province from June 1， 2012 to May 31， 2013， the temporal variations of the geomagnetic harmonic wave amplitude ratios were analyzed. It was found that both the geomagnetic harmonic wave amplitude ratios Y_ZHy and Y_HD were enhanced just before the M_S5.5 Eryuan earthquake on 3 March 2013， but no similar variation appeared for the ratio Y_ZHx. In order to explain the result， both the temporal evolution of original vertical components (Z) and the distribution of Weiss vectors were analyzed， and it was found that there was an NS-oriented high conductivity layer on the underground. Considering that the ratios Y_ZHy， Y_HD and Y_ZHx are related with the variation of NS-oriented conductivity， the heterogeneity of underground electrical structure and the variation of EW-oriented conductivity， respec-tively， it was deduced that the variation of the geomagnetic harmonic wave amplitude ratios was closely related to the underground electrical structure.