Herein, we study the out-of-lock frequency-tracking problem in a resonant fiber-optic gyro (RFOG). First, we analyze the reason and mechanism of out-of-lock frequency tracking, and find that the change of the current in frequency-tracking synchronization and the symmetry change caused by non-reciprocal noises, such as backscatter and polarization coupling, are the main reasons for the peak pulse and zero-bias change, respectively. Second, we propose a scheme for out-of-lock frequency-tracking control of the RFOG based on temperature closed-loop feedback of the semiconductor laser. The long-term tracking synchronization of the laser central frequency with the fiber resonator's single-resonant frequency can be realized by temperature closed-loop control; thus, the gyro output error caused by out-of-lock frequency tracking is eliminated. The overall technical scheme, signal processing flow, and implementation method of out-of-lock control are described in detail. Finally, we construct a successful RFOG prototype and test the static performance of the RFOG both before and after implementing the out-of-lock control. The test results show that this out-of-lock frequency-tracking control scheme can reduce the output pulse amplitude mutation of the RFOG from 3000 (°)/h to 200 (°)/h and the output zero-bias change of the RFOG from 600 (°)/h to 0 (°)/h, which completely eliminates the zero-position change in the frequency secondary locking process; consequently, the gyro precision is significantly reduced to 4.9 (°)/h (for 100-s smooth integration time).