The measurement of electromagnetic field at two sites in Hanwang and Chouziba for monitoring aftershock series has been done in continuous observation after the M_s8.0 Wenchuan earthquake on May 12, 2008. The measurement was done in two periods. Recorded data covered 35 aftershocks with magnitude more than M_s4.0. Comparing with the seismic data recorded at Hanwang station for 15 events, the coseismic signal of aftershocks exists on all components of the magnetic and electric fields at two different sites. The EM coseismic signals arrived simultaneously with the seismic waves,but not at the origin time of the earthquake. The signal produced at the origin time of the earthquake is much smaller than coseismic signal of the waves. It can′t be identified from the time series. In addition, the generation of coseismic electromagnetic signals can be explained by earthquake dynamo mechanism. Analysing and comparing the relationship between the seismic displacement and EM coseismic signals，the mechanism of EM coseismic signal generation appear to be driven by seismic waves. That is，the EM observation system set up in geomagnetic field moved with the ground motion so that the EM coseismic signals were produced. The observations of seismic displacement and electromagnetic signals verified it.