Abstract: On the basis of measring result of horizontal and vertical distribution of residual stress field in four stfong earthaquake zones in southwest China and experimental result,this treatise has discussed the acting of paleotectonic residual stress field on the focus:reducing strength of focal rock mass, increasing deformation of focal rock mass, facilitating earthquake with the superposing field of residual and present stresses, releasing residual elastic energy in rock mass breaked and adding it to present elastic energy released by focus to raising earthquake magnitude and activity.