The invention relates to a portable setting instrument for fixed values of ring main units and a use method thereof, and belongs to the technical field of electrical measurement equipment. According to the setting instrument, the inner portion of a shell is divided into two layers of structures, wherein a single chip microcomputer system module of the lower layer and the bottom of the shell, a keyboard control module of the upper layer and the single chip microcomputer system module of the lower layer, and the keyboard control module of the upper layer and the top of the shell are separated through supporting columns by distances for heat dissipation respectively; a battery is arranged between the single chip microcomputer system module and the bottom of the shell; basic information of thedifferent ring main units is stored in the single chip microcomputer system module. The method comprises the steps of program prefabricating, input inquiring, fixed value setting and remote storage.The instrument has good heat dissipation performance and is convenient to carry, convenient and practical; by means of the use method, the fixed values of the ring main units can be conveniently and fast set, so that it is convenient for a worker to learn about the running situations of the ring main units in real time; during worker replacement, a subsequent worker can conveniently know the situations of the ring main units of a circuit at any time through the instrument.