Lateral buckling and local buckling are two important influential factors for bearing capacity of a steel-concrete composite beam. The existing codes of a steel-concrete composite beam generally use steel structure design methods to calculate stability of a composite beam, and the results do not agree with the fact. This paper analyses global lateral buckling forms of I shape steel concrete composite beam and a calculation model of I shape composite beam stability analysis is presented. The critical moment of lateral buckling of the composite beam compressive bottom flange in the hogging bending moment region is derived in accordance with computation module. The author also studies the mechanical property of the steel web under combined action of bending stress, axial compressive stress and shear stress in the hogging bending moment region. In the light of correlation equation under combined eccentric compression and shear force, an elastic buckling factor of the steel web in complicated stress state is obtained. Based on buckling analysis results, a height to thickness ratio of steel beam in the elastic strained stage without transverse stiffening rib is proposed. Compared with existing stability theory and calculation method of I shape composite beam, correction methods and advices of stability design for I shape composite beam in the hogging bending moment region are presented.