The invention relates to a device for detecting the circumferential gap of elastic rings. The device comprises a bracket, wherein, a detecting ring driven by a driving motor is rotationally mounted on the bracket; the inner ring surface of the detecting ring is used for mounting a part to be detected; a photoelectric sensor group is mounted in a position right opposite to the juncture of the inner ring surface of the detecting ring and the part to be detected on the end side of the detecting ring; the luminescence unit and the photoelectric receiving unit of the photoelectric sensor group are placed on the two end sides of the detecting ring respectively; the driving motor is fixedly arranged on the bracket; and an encoder is in transmission connection with the output shaft of the driving motor. The device can automatically measure and visually display in the form of polar coordinate the position of the light leakage part of the elastic rings, such as a piston ring, and the size of the gap between the elastic rings; the measurement result is in good corresponding relation with the measurement result of the detection method stipulated by the current Chinese Standard; and the transformation from the manual visual measurement of light tightness to the measurement dependent upon instruments and the transformation from qualitative judgment to quantitative judgment are realized, therefore, the device has the advantages that the measuring result is accurate; the coherence is good; the structure is simple; and the use is convenient.