The relationship between the electronic structure of FCC phase in Fe-Mn-Si alloy and itsstability has been studied by using the discrete variational method based on the first principle. The rea-son why Mn and Si elements have different influences on the stacking fault energy may be related tothe electron concentration ( e/a). Si reduces the hole number of 3d band while Mn is rather complicat-ed. The binding energy has been calculated and the experimental results that martensite start tempera-ture (Ms) varies with Si and Mn are explained. When the external stress is exerted in three directions,the electronic structure, the total density of states, the energy gap at Fermi energy level( EF) and theenergy degeneracy will change into other states. When the different external stresses are exerted inone direction, 3d or 4s orbital occupations of the central atom decrease, the partial density of statesseems to be thinner and its peak increases at EF, the bond orbit shrinks in the direction of the externalstress and another bond orbit comes out vertically. These lead to an a decrease in the structural stabili-ty and an increase in Ms temperature under the external stress.