Using the initial phase, breakout phase and stopping phase of long period P-wave records of WWSSN, we have studied the source rupture process of the 1976 Songpan great earthquakes (Aug. 16, M=7.2; Aug. 22,M=6.7; Aug. 23, M=7.2), and got the dynamic parameters of sources of the three great earthquakes. The length of initial rupture area for the three great earthquakes is 8.5 and 6km respectively, calculated by the weaker initial, phase of P-wave. All of the three great earthquakes is two-side rupture, the rupture length of source body is 73, 34 and 54 km respectively.
The result shows that in the source regions, the three great earthquakes exist a part of superposition phenomena, so total source dimension is only 90 km; and the source depth is 9km determined by the breakout phase of the Songpan great earthquake of Aug. 16. It is also found that the obvious stopping phase and quicker rupture speed of source fault are related to the adjustment element located at the ends of source region.