The invention provides a universal measuring tool and a measuring method of the diameter of a trapezoid packing ring groove. The measuring tool is composed of a vernier caliper with a watch and measuring auxiliary pieces. Two measuring auxiliary pieces are symmetrically arranged on a measuring claw under the frequently-used vernier caliper with the watch, and two protruding spherical measuring heads are symmetrically arranged under standard connection blocks of the measuring auxiliary pieces. In the measuring process, the lower faces of the two standard connection blocks are attached to the flat planes of flanges to be used as measuring standards. The side faces of ball heads of the two spherical measuring heads are respectively in contact with inclined planes of two corresponding sides of the trapezoid packing ring groove. A direct measuring value of the vernier caliper with the watch is read, a relevant correcting value is added or subtracted, so that the value of the diameter of an outer circle or the diameter of an inner circle of the measured trapezoid packing ring groove is obtained. The measuring tool has the advantages of being simple in structure, visual in value reading, strong in universality, and convenient to use. The detection time of the trapezoid packing ring groove can be shortened, and the measuring accuracy of the trapezoid packing ring groove is improved.