The lock-in threshold can be obtained by reducing the rotation rate. The lock-in threshold obtained using this method is inconsistent with the angle random walk derived from Allan variance. A dynamic burn-in grating model is proposed. The burn-in effect happens in the mirror not only in the standing wave state, but also in the traveling wave state, and it responds dynamically to beat frequency, which is different from previous research results. It can be derived that the lock-in threshold increases with the decrease of rotation rate for this model, and many puzzling phenomena of the ring laser gyro can be explained.