The aluminum alloy gusset joint has a good global stiffness performance, so the out-of-plane initial bending stiffness interaction of members cannot be neglected. Based on the existing research, the matrix for the bending stiffness of aluminum alloy gusset joint was defined, and the finite element (FE) analysis was developed. Fourteen FE models of aluminum alloy gusset joint were established to discuss the bending stiffness interaction of members, taking into account the thickness of plate, the radius of plate, the height of member’s cross-section and the number of bolts. According to the FE results, the theoretical formulae to estimate the bending stiffness interaction of members on the aluminum alloy gusset joint were proposed. Making a comparison of the theoretical results with FE results, the theoretical results have a good agreement with the FE results, which verifies the applicability of the proposed formulae.