Abstract: A two dimensional non-linear finite element scheme was used in the study of stress field on a deep section of a typical graben system. The parameters of the specific section is determined according to the data given in ref,The programme employed is a transplantation of the programme employed by Wang Ren. The faults are represented by laminar material, transversely isotropic elasto-plas-tic material. The change in the degree of safety caused by the upwarping of mantle is estimated (See Figs 2 and 3).The concept of degree of safety has been employed in R. Wang's paper. This is a non-dimensional quantity, characterizing the degree of safety of a material under Coulomb shear criterion. The calculation equation is as follows: α=(μσ_n+|τ|)/(μ·σ_n) For the fractured zone, σn and τ are the normal stress (trensile as positive) and shear stress on the fault plane, respectively.μ is the coefficient of friction. For the elastic area outside the fault, the value of degree of safety α is calculated along the direction which forms an angle of intersection of 0.5Arctg (1/μ) with the maximum principal compression stress. It is easy to see thatwhen α≤0, shear fracture occurs in the material;when 0<α≤1, the larger α (nearer to 1), the higher safety of material;when 1<α, then σ_n>0, this plane is under tensile stressThe upper surface is a free boundary. The two sides are under compressive stresses which increase linearly with depth. For the base boundary two cases are computed: the displacement in the vertical direction is zero or non-zero. The upwarping mantle can be described by non-zero displacement in the vertical direction of base boundary. By comparing two results calculated, the danger of fracture caused by the upwarping of mantle is obtained. Let f and f₁ be the lithostatic pressure and compressive stress acting on two sides of the boundary, respectively. The Fig. 3 and 2 are the diagrams of the change of safety degree caused by upwarping of mantle when f₁ = f and f₁ = f/2, respectively. The safety degree increases in the dotted areas. The results of computation coincide with the following characteristics of seismicity in graben system: a) the graben system is an area of intense seismicity, but much intensive is its inner part as compared with the outer part; b) with only individual exception, the foci of strong shocks are shallow.