Whispering gallery mode resonators (WGMRs) are used as excellent optical feedback components of narrow-linewidth fiber lasers, applied from distributed fiber sensing to optical fiber communication. However, WGMRs lead to output of a few microwatts and serious multi-modes in lasers. In this Letter, we fabricated the specially designed WGMR with an over-coupling structure, and its quality (Q) factor was over 10⁹. It improved laser output power significantly. Based on that, dynamic population gratings were applied successfully in the laser. Finally, a single-frequency WGMR fiber laser was realized. Its linewidth was less than 1.07 kHz, its output power was over 0.107 mW, and its spectral signal-to-noise ratio (SNR) was nearly 50 dB. Our research offers a new scheme of a single-frequency narrow-linewidth WGMR fiber laser.