On 12th of May, 2008，an M_S8.0 earthquake occurred in Wenchuan county, Sichuan province of China. We configured a generalized array with the broadband seismic stations of the Alaska Regional Network，USA, and imaged the temporal and spatial variation of high-frequency (>0.1 Hz) energy radiation sources by means of one of the non-plane wave array techniques—migration stacking technique. The image shows that the earthquake faulting initiated at the instrumental epicenter and extended north-eastwards about 300 km, the whole rupture process lasted for at least 90s, and the average rupture speed was about 3.4 km/s. The whole process may be divided into two major periods. The first period is 50 s and in this period the rupture propagated about 110 km, with an average rupture speed of 2.2 km/s; the other period is 40 s and in this period the rupture propagated about 190 km, with an average speed of 4.8 km/s. It seems that supershear rupture appeared in the later process of the earthquake, and the second segment might be dynamically trigged by the first one.