Abstract: Based on the electromagnetic data recorded by DEMETER satellite during the past ten days before 12 May 2008 WenchuanMS8.0 earthquake, an anomaly below cutofffrequency is detected at the sixth day before the earthquake using power spectrum analysis. In order to learn the propogation characteristic of the abnormal electromagnetic emmision, singular value decomposition (SVD) method is used to get polarization and wave vector of the emmison signals. The result shows that the anomaly reveals plane wave features and it propagated upwards. No similar phenomena are found above other nonseismic regions.