The invention relates to a transition joint of a steel part. The transition joint comprises stiffening angle steel, angle steel main materials and a joint plate perpendicular to the angle steel main materials. The transition joint is suitable for large-load rod tower structures with a high voltage grade or with multi-circuit transmission lines on same tower in electric transmission lines or unequal sections in other industry fields; and by means of design and modification of a traditional transition joint, the aims of optimizing structural arrangement, increasing the joint rigidity, decreasing the wind stopping area, improving the mechanical performance of local bending prevention and wind resistance of the structure, saving materials and improving the machining efficiency are achieved.