Abstract: A damage model of steel is built on the basis of the theory of plastic strain and energy dissipation, in which the damage accumulation effect and the strain strengthening effect are considered. With this model, the elasto-plastic element stiffness matrices and the structural dynamic equilibrium equations of spatial trusses are derived. The corresponding computing program of the finite element analysis is developed, and the numerical simulation of an example truss is carried out. Numerical results show that the elasto-plastic seismic responses of the spatial trusses are significantly affected by the damage accumulation effect. The stresses of the damaged rods increase and its strains decrease, but both stresses and strains of the undamaged rods increase. Under the combined excitations of the earthquakes in different directions, the variational tendency of the internal forces and deformations of the truss is the same and irrelevant to the damage accumulation effect. Under the excitations of different earthquakes, the damage accumulation effect on the internal forces and deformations of the truss is also significant.