The invention relates to a variable diameter steel structure spiral ramp, also relates to a construction method of the variable diameter steel structure spiral ramp and belongs to the technical field of construction engineering. The spiral ramp comprises oblique columns and segmented connection ramp units; the oblique columns support the segmented connection ramp units; the oblique columns are formed by base anchors and inclined columns; the inclined columns are welded to the base anchors; the ramp units are provided with top plates which form into partial spiral ramp surfaces; the number of the oblique columns is at least three; the oblique columns are uniformly fixed on the external side of the spiral ramp with upper ends incline outward; cantilever supports which horizontally penetrate the cross section of the ramp unit are arranged on the middle of the ramp unit which is adjacent to the inclined column; the cantilever supports and connection arm end sockets are fixedly connected through welding to form into connecting column ramp units; the connecting column units and transitional ramp units are sequentially fixedly connected through welding to form into the integral variable diameter ramp with the external diameter gradually expands from bottom to top. The spiral ramp has the advantages of being reasonable in structure, ideal in rigidity and intensity and convenient to construct, greatly shortening the construction period, being practical and guarantee engineering quality.