A linear inversion technique based on assumption of constant rupture velocity and fixed source time functions of sub-events is proposed in this paper for inverting teleseismic waveform data to image spatio-temporal variation of source mechanism of large earthquakes, and applied to the 2008 Wenchuan M_S8.0 earthquake, obtaining its image of the spatio-temporal variation of source mechanism by inverting 48 worldwide stations of long period waveform data. The image shows that the mainly thrust mechanism appeared in the southwest segment of the fault, and the strike-slip component was getting more and more with the fault extending northeastwards, with the turning point at which the strike-slip component became larger than the thrust-slip component appearing around 190 km northeast to the epicenter. In order to inspect the validity of the used method and reliability of the inverted results a specially designed numerical test was performed. The inspection indicated that the inversion method proposed in this paper was valid and the inverted results regarding the great Wenchuan earthquake were trustable (except those on one segment where the long-period component is weak). A comparison shows that the inverted results were coincident with the field-investigation results.