The invention relates to a device for measuring the circumferential strain of a rock specimen. The device comprises a first fixed seat and a second fixed seat, wherein the first and second fixed seats are arranged on the two sides of the surface of the upper part of the cylindrical rock specimen to be measured respectively; a cambered force bearing plate is arranged at the bottom of the side, close to the second fixed seat, of the first fixed seat, the second fixed seat is mounted on the force bearing plate, and the first and second fixed seats are fixedly connected through an elastic connecting piece; the first and second fixed seats are tightly hooped on the periphery of the rock specimen to be measured through a hooping band; a linear displacement sensor is mounted on the first fixed seat, a circular deformation displacement pushrod is fixed on the second fixed seat, the linear displacement sensor points to the circular deformation displacement pushrod, and the detection end of the linear displacement sensor is pressed against the end part of the circular deformation displacement pushrod; the linear displacement sensor is externally connected with a background computer. The device has the advantages of simple structure, convenience for mounting, capability of comprehensively measuring the radial deformation of a rock and relatively high accuracy of a measurement result.