Large laser gyro is an inertial sensor based on Sagnac effect.As the size increases, the fundamental mode operation and gain become important parameters. Aiming at lens matching, discharge current size and cavity length, the characteristic parameters of ABCD matrix and Gaussian beam are calculated, and the influence of the waist spot position of the ring cavity and the discharge current on the diaphragm under different side lengths are verified in this paper.The results show that the quality of the light spot at the light threshold is the best. At this time the light spot is the flattest, and the ratio is close to 1. The symmetrical structure is conducive to efficient signal reading.This has certain reference significance for the cavity design of large laser gyro.