The invention relates to a steel structure profile roundness evaluation auxiliary measuring arm, a calibration method and use method thereof. The auxiliary measuring arm is provided with a rigid straight rod and a connection head; the connection head is fixedly connected with the front end of the rigid straight rod; the front end portion of the rigid straight rod is a magnetic connection portion; the front end surface of the magnetic connection portion is fixedly connected with a magnetic material; the main body of the external end surface of the magnetic material is a flat surface; and the outer end surface of the magnetic material is flushed with the front end surface of the magnetic connection portion or protrudes out from the front end surface of the magnetic connection portion. With the above technical schemes of the invention adopted, the auxiliary measuring arm can satisfy measurement requirements. Two reflecting sheets are pasted on the rigid straight rod; and the key calibration dimensions of the auxiliary measuring arm can be calibrated according to the coordinates of the two reflecting sheets, and the coordinate values of a blind point to be measured can be also obtained through calculation. The structure of the auxiliary measuring arm of the invention is greatly simplified than that of an existing measuring arm; and the auxiliary measuring arm has no harsh special requirements for processing and manufacturing. With the measuring arm adopted, the coordinate values of a point to be measured can be calculated conveniently.