On 12 May 2008, the Mw 7.9 Wenchuan earthquake occurred along the Longmen Shan fault in the northwestern Sichuan province of China, resulting in huge damage as well as loss of human lives. Rapid estimation of the rupture process is essential for emergency response. In this paper, the relative back-projection method based on nonplane-wave array technology was applied to reconstruct the rupture process of this earthquake. First, the principle of the relative back-projection method was briefly introduced. Then the method was used to obtain the rupture process using the broadband data of 16 stations from the Australian National Seismic Network from the Incorporated Research Institutions for Seismology (IRIS) Data Management Center. The results showed that this earthquake ruptured mostly unilaterally along the Longmen Shan fault. Its rupture time was about 98 s, rupture length was about 290 km, and mean rupture velocity was about 3.0 km/s. Starting from the initial rupture point, epicenter (31.00° N, 103.32° E), the great earthquake ruptured northeastward along the Longmen Shan fault with two major energy releasing peaks. One was located about 50 km northeast of the epicenter at about 20 s after the earthquake began and the other about 140 km northeast at about 50 s, which coincided with macrodamage at those two places.