Abstract: To investigate external causes of motivating earthquake on Wenchaun in Sichuan province, P.R.C, the relationships between the Wenchuan earthquake, in Sichuan province, P.R.C, and impoundage in Three Gorges reservoir were presented for discussion in this paper.The relationships included:Distance and geometry mode between the Longmenshan fault and the center of gravity of large impoundage capacity in Three Gorges reservoir, effects of large impoundage capacity in Three Gorges reservoir, the acting result of westward horizontal thrust transmitting through hard and integral plate of Sichuan Basin, time factor between strong tectonic earthquake and impoundage elevation.