To investigate the overall structural stability of the steel heavy truss and obtain stability coefficient under vertical loadings, the finite element models were established using ABAQUS program. The nonlinear analysis was carried out by introducing the initial geometric imperfection of the steel truss obtained through the linear elastic buckling analysis. The effect of the out-plane slenderness ratio of top chord on the stability of the steel truss was discussed to form the stability coefficient formula. Results show that the stability coefficient increases faster with the decreasing of the out-plane slenderness ratio of top chord, and buckling failure transits to strength failure when the out-plane slenderness ratio reduces to a specific value. Furthermore, the influence of steel strength, unequal sections of top and bottom chords, loading positions and depth-span ratios were studied, and the revised stability coefficient formula was suggested by a selected enveloping curve.