Based on seismic wave records of the Chengdu digital seismic network and Zipingpu reservoir digital seismic network from August 16, 2004 to May 12, 2008 when the Wenchuan Ms8. 0 earthquake occurred, the parameters of focal mechanisms of 486 earthquakes with magnitude larger than M1.6 in the Zipingpu reservoir area were calculated using amplitude ratio method, meanwhile the temporal-spatial variation characteristics of mechanisms and stress field were analyzed based on these parameters. Results show a low ratio of thrust earthquakes and an increased number of strike-slip earthquakes in the reservoir water area in the period from 2006 to 2008. While in the areas far from the reservoir waterfront, the thrust earthquakes took up a high proportion and the strike-slip ones did not increase. The direction of mean principal compressive stress field was deflected and disturbed differently in each area before the Wenchuan Ms8. 0 earthquake.