The precision anomalies in variable-temperature experiments are accounted for more than two thirds of the faults in the total reflection prism laser gyro. Because of the prism machining error, the assembling error, the pyramidal error of the cavity and the improper adjustment, the actual optical path in ring laser may deviate from the ideal path in the meridian and sagittal planes, the high-order transverse modes and the fundamental mode are mixed and oscillated simultaneously, it will lead to the precision of the gyro falls or even the faults. In this paper, based on the laser mode theory and the operation characteristics of the ring laser, combined with the movement rules of the optical path in the prism ring resonator under the temperature variation conditions, the mechanism of the fault gyro interference spots distortion in the variable-temperature experiment is investigated. Theoretically, according to the optical characteristics of the prism ring resonator, considering the multiple high-order transverse modes mixture, the physical model of the two-beam interference is established. The numerical simulation shows that the energy distribution of the fundamental mode interference field is uniform, and the dark stripes are equally spaced, which is suitable for the signal source for the interferometry. The edge of the TEM11 mode interference field is clear, but the contrast in the middle of the interference field is weak, which is not conducive to signal detection. The energy of the interference field of the TEM22 mode is dispersive. The parts of the interference field are distorted, which caused the gyro to fall in the variable-temperature experiment.In the experiments, the cat’s eye laser is used to realize the independent oscillation of the high-order transverse modes. On this basis, the two-beam interference device is used to form a ring cavity. The interference spots of the high-order transverse modes are obtained at the 2o interference angle. The theoretical analysis is verified by the experiments. In the precision experiments of the laser gyro under the variable-temperature conditions, the laser gyro is placed in a heating block. The temperature is changing from 25℃ to 70℃ at the rate of 1℃/min. The interference spot is led out from the heating block by the fiber that is connected with the self-focusing lens. The variable experimental results show that the interference spot of the fault gyro is distorted discontinuously. The precision of the gyro is intermittent normal and repeated abnormal. On this basis, combined with the variational characteristics of the optical path in the prisms ring cavity under the variable-temperature: the optical path is diffused outward at the high temperature and shrinked inward at the low temperature, the causes of the gyro failure are analyzed. For stable limiting the high-order transverse modes oscillation, the external aperture is assembled around the clear aperture. For example, when the optical path has inclined slight displacement in the sagittal plane because of the temperature variation, it can be solved by replacing the single external aperture to the oblique 45° bilateral external aperture. The variable-temperature fault of laser gyro is effectively solved by the above repairing methods.In the conclusion, the precision anomalies of the total reflection prisms laser gyro in variable-temperature experiments are caused by the multiple high-order transverse modes oscillating simultaneously. The reason of the gyro failure is that the modes limited structure cannot keep the fundamental mode oscillating individually after the optical path is changed. The multiple transverse modes are oscillated simultaneously, the boundary of the interference spot is fuzzy, and the interference dark stripes are not suitable for being detected. This phenomenon seriously affects the operation of the laser gyro. The pertinence assembling using an external aperture can solve this kind of fault effectively.