The resonator integrated optic gyros (RIOGs) based on the Sagnac effect have gained extensive attention in navigation and guidance systems due to their predominant advantages: high theoretical accuracy and simple integration. However, the problems of losing lock and low lock-in accuracy are the bottlenecks, which restrict the development of digital RIOGs. Therefore, a multilevel laser frequency lock-in technique has been proposed in this Letter to address these problems. The experimental results show that lock-in accuracy can be improved one order higher and without losing lock in a variable temperature environment. Then, a digital miniaturized RIOG prototype ($18\mathrm{cm}\times 18\mathrm{cm}\times 20\mathrm{cm}$) has been produced, and long-term (1 h) bias stability of 26.6 deg/h is successfully demonstrated.