Overall gusset plate due to its advantages in the design, manufacture, installation, widely used in large span steel bridge, but for the whole gusset plate of local stress mechanism few scholars study. With the development of computer technology, often in practical projects through the finite element software to simulate, domestic scholars about the boundary conditions of the simulation is roughly divided into three kinds, that is, one end of the consolidation, center consolidation and simply supported at both ends, the principle of selecting the three boundaries often do not mention, for later users bring distress, In this paper, through theoretical analysis and finite element software simulation, illustrates the principle of three kinds of boundary selection, And according to the viewpoint of stress nephogram real simulation presents a recommended boundary conditions which formed at both ends simply supported constraints.