The invention discloses a locating ring with internal and external diameters being freely adjustable within a certain range and a mounting method of the locating ring. The locating ring is characterized by being composed of an upper thickened ring and a lower thickened ring; a plurality of spring pieces which are bent towards the outer side, a plurality of long adjusting bolts and fixing nuts are arranged between the two thickened rings; and each thickened ring is provided with a plurality of radial mounting screws. The stretching and compression for the spring pieces which are bent towards the outer side are realized by the adjusting bolts and the fixing nuts, and the external diameter of the locating ring is freely changed within a certain range, so that the locating ring has a function of adapting to various apertures; and the aims of changing the internal diameters of the thickened rings as well as mounting and fixing an internal tubular instrument are achieved by changing the lengths of the mounting screws extending into the thickened rings. By using the locating ring, the defects that the instrument in the prior art is relatively difficult to locate in the center of a hole and is relatively difficult to mount after being provided with an auxiliary locating device are overcome. The locating ring is simple and convenient to operate, wide in application range and high in utilization ratio.