The invention discloses a device for improving a fiber stimulated Brillouin scattering threshold of two rare earth ultra-magnetostrictors and belongs to the field of high-power narrow-linewidth fiber lasers and narrow-linewidth fiber amplifiers, and the device can be used for solving the defects of high requirements, short length of adjustable fiber, uncontrollability and the like in the prior art. The device for improving the fiber stimulated Brillouin scattering threshold of the two rare earth ultra-magnetostrictors comprises a first rare earth ultra-magnetostrictor (1), a second rare earthultra-magnetostrictor (2), an alternating current source (3), a spiral coil (4) and a fixed base (5); the first rare earth ultra-magnetostrictor (1) and the second rare earth ultra-magnetostrictor (2) are arranged in the groove of the fixed base (5); the spiral coil (4) is sheathed at the exteriors of the first rare earth ultra-magnetostrictor (1) and the second rare earth ultra-magnetostrictor (2) and then laid on the fixed base (5); and the alternating current source (3) is connected with two ends of the spiral coil (4). The device has a wide adjustment range and is controlled by the alternating current source.