In this paper, we investigate the performances of an in-house fabricated confined-doped active fiber in the applications of all-fiber high-power single-frequency amplifiers. A 210-W single-frequency single-mode fiber laser is obtained directly, which confirms the excellent performance of the confined-doped active fiber for high-power single-mode operation. To further demonstrate the power scalability of the fiber amplifier, the strategy of applying a temperature gradient along the active fiber is investigated numerically and experimentally, and an up to ∼75% enhancement of the stimulated Brillouin scattering (SBS) threshold is achieved. As a result, a 368-W single-frequency fiber laser is obtained with the beam quality factor of Mx2 = 1.19, My2 = 1.26. Overall, the technique of the confined-doped active fiber provides a promising approach to scale the output power of single-frequency single-mode fiber lasers.