The invention provides a Z-3 type under-reinforced encased concrete sectional steel special-shaped column, and belongs to the technical field of structure engineering. A component uses a sectional steel encased concrete form, wherein the encased concrete is provided with longitudinal erection steel bars and hoop bars and does not participate in the longitudinal stress; the local stability of a special-shaped column plate element can be limited; the sectional steel dimension is reduced; the problems of poor fireproof performance, easy rusting corrosion and the like of a steel structure buildingcan be effectively solved; the dimension of the encased concrete is smaller, so that the cross section of the component cannot be greatly increased; and the assembly and the restoration among modulescan be performed according to requirements. Through the welding of a steel bar cage and sectional steel, the effective bonding of the encased concrete and the sectional steel can be effectively ensured. The encased concrete sectional steel special-shaped column is processed and completed in a factory. The connection of upper and lower columns is only completed through sectional steel welding connection or screw bolt connection; and after the connection is completed, encased concrete is added in the connection position. The Z-2 type under-reinforced encased concrete sectional steel special-shaped column has the advantages that the industrialization degree is high; the integration of design, production and construction is realized; the residence industrialization level is improved; the component realizes the field assembly through hanging; and the construction period is short.