The utility model discloses an iron-tower variable-cross-section built-up joint, and the joint comprises a cross-shaped connecting plate. The upper part of the cross-shaped connecting plate is connected with double-combination main materials, and the lower part of the cross-shaped connecting plate is connected with four-combination main materials. In use, the double-combination main materials are arranged in a corner-to-corner manner to form a cross shape and are disposed at the upper part of the joint. The four-combination main materials are disposed at the lower part of the joint in a back-to-back manner. The double-combination main materials and the four-combination main materials are symmetrically disposed on the joint, and the gravity centers of the double-combination and four-combination main materials are overlapped, thereby avoiding problems, simplifying the structure, reducing the cost, and enabling the joint to be easy to make. Meanwhile, the weight of an iron tower is reduced, and the transition from the double-combination main materials to the four-combination main materials is natural and transition parts are beautiful. The utility model also discloses the iron tower.