From the Point of view of the focal mechanizm and behaviour of grou-nd crack,the component of right-lateral shear strike slip was dominant and at the same time with dip slip during Tangshan earthquake.This sho-wed that the event had occurred by reason of horizontal and vertical force.Inthis paper the authors use three dimensional finite element method to compute the stress field of Tangshan area.It is believed that there are two kinds of distributive force:body force and surface force.And the latter force may be classified into horizontal direction and vertical direction.Horizontal stress field changes with the depth,and maximum horizontal compressive stress is equal to 3 times of hydrostatic pressure,the depth being equal.Its orientation is N85°E.The vertical stress is equal to 1.2 times of hydrostatic pressure results of computation are in the main con-sistent with the occurrence of the event.Tangshan earthquake occurred per-haps by reason of both horizontal stress field and vertical stress field,the former being dominant.