The invention relates to an assembly type steel pipe casing reinforced concrete combined joint and a mounting method, and belongs to the technical field of structure engineering. The traditional column beam has problems that the welding quality is unstable and the construction time is long due to existence of a steel tube welding link in a construction process. The invention greatly solves the problem. The assembly type steel pipe casing reinforced concrete combined joint comprises a high-strength outer-coating steel tube, a reinforced steel bar limiting plate, high-strength reinforced steel bars, conversion spacing casings and a common outer-coating steel tube, wherein the conversion spacing casings are arranged at the upper end and the lower end of the common outer-coating steel tube, the conversion spacing casing at the upper end of the common outer-coating steel tube is connected with the high-strength outer-coating steel tube, and the conversion spacing casing at the lower end of the common outer-coating steel tube is connected with the high-strength outer-coating steel tube; the high-strength reinforced steel bars penetrate through the whole common outer-coating steel tube, the upper ends and the lower ends of the high-strength reinforced steel bars are communicated with the interior of the high-strength outer-coating steel tube, the high-strength reinforced steel bars are connected with the conversion spacing casings, and the upper ends and the lower ends of the high-strength reinforced steel bars are connected with the reinforced steel bar limiting plate. The assembly type steel pipe casing reinforced concrete combined joint and the mounting method is applied to building column beams in a building structure.