The Indonesia region is one of the most seismically active zones of the earth. On December 26,2004 an M_W9.0 earthquake occurred off the west coast of northern Sumatra Indonesia,and is also the largest earthquake in recent forty years. The epicenter is located in the east Sunda trough. The trough is the boundary of the Australian plate and the India plate. This paper selects the earthquake aftershock distribution range space as the study area,which is divided into two parts of north and south. The south latitude is 11°S~5°N,and the north is 5°N~15°N. Based on the global focal mechanism directory of the China Earthquake Information Network,we analyze the focal mechanism of the area,and use the consistency parameter a of focal mechanism and tectonic stress field to conduct the earthquake test. The research results show that before the M_W ≥ 7.5 earthquake,there was a lower consistency parameter a. Before the M_W9.0 earthquake,the a value is in a turbulent state. The study region is thousands of kilometers,and focal mechanism and tectonic stress field of the continuous occurrence earthquake of M_W ≥ 5.0 is consistent which are not random phenomena,but with the precursory phenomena to be of some prediction significance.