For an Er-doped fiber laser, for the first time, to the best of our knowledge, we demonstrate both experimentally and theoretically a novel mechanism of harmonic mode-locking based on the electrostriction effect leading to excitation of the torsional acoustic modes in the transverse section of the laser. The exited torsional acoustic modes modulate the fiber birefringence that results in synchronization of oscillations at the harmonic modes and the linewidth narrowing with the increased signal-to-noise ratio of 30 dB. By adjusting the in-cavity birefringence based on tuning the polarization controller, we enable the selection of the harmonic mode to be stabilized.