In this Letter, we present a high-strain resolution fiber laser-based sensor (FLS) by a novel optical phase-locked loop (OPLL) interrogation technique based on a root mean square detector (RMSD). The sensor consists of a distributed feedback (DFB) fiber laser as a master laser for strain sensing and a fiber Fabry-Perot interferometer (FFPI) as a reference. The laser carrier locks to the reference by the PDH technique, and the single sideband laser working as a slave laser locks to the DFB sensing element using the OPLL technique, respectively. A strain resolution of 8.19 pε/√Hz at 1 Hz and 35.5 pε in 10 s is achieved in the demonstrational experiments. Significantly, the noise behaves a 1∕f distribution below 0.2 Hz due to the very low pump power for the DFB sensor and an active thermostat testing environment. The proposed OPLL interrogation brings new thinking for the demodulation of FLS. This strain sensor based on FLS has a great performance in strain measurement and can be a powerful tool for geophysical research.