Whispering gallery mode resonators (WGMR) 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 the work, we fabricated the especially designed WGMR with an over-coupling structure, and its 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, output power was over 0.107 mW and spectral SNR was nearly 50dB. Our research offers a new scheme of single-frequency narrow-linewidth WGMR fiber laser.