We study the initial rupturing process of the 2008 Mw7.9 Wenchuan earthquake occurred on the eastern Qinghai-Tibetan Plateau, using the back-projection method and the P-wave phases recorded by a near-source seismic array deployed in the Zipingpu reservoir region. We first use the P-wave first arrivals and the back-projection method to relocate the initial rupturing position of the Wenchuan earthquake. The results show that the initial rupture of the Wenchuan earthquake is located at 31.014°N ± 0.003° and 103.391°E ± 0.003° with a focal depth of 8.4 km ± 0.82 km, at 14:27:58.800 ± 0.4 s on 12 May 2008 (hereafter, the time mention are Beijing time = GMT + 8). Then we use the back-projection method to obtain a detailed initial rupture process in the three-dimensional space surrounding the relocated hypocenter. The rupturing images indicate that the initial rupture propagated downward and southwestward firstly and then it moved upward and northeastward with an average speed of 2.7 km/s in the initial stage. The deduced initial rupturing process of the Wenchuan earthquake is consistent with the pre-slip model and the depth of the nucleation zone was around 7.5–9.5 km.