An external-optical-cavity-based laser frequency-locking method with second-harmonic demodulation was proposed, analyzed, and demonstrated. The second-harmonic component of the cavity output was demodulated to feed back to the frequency-locking loop, resulting in a high sensitivity, great carrier suppression, and large modulation bandwidth. The experimental demodulation curve was consistent with the simulation result. A distributed feedback fiber laser was then locked using this technique. A carrier wave suppression ratio of -67 dB and a laser frequency noise floor of 1 Hz/Hz level above 1 Hz were achieved. This technique has great potential to be used in resonator optic gyroscopes.