A watt-level ultrahigh optical signal-to-noise ratio single-longitudinal-mode (SLM) tunable Brillouin fiber laser (BFL) has been demonstrated. By optimizing the length of the single mode fiber (SMF) cavity at 11 m and its output ratio at 60%, 1.04 W output power, as well as stable SLM operation is obtained at 2.24 W pump power. The single pass cavity BFL has the advantage that the Brillouin pump frequency does not need to match the cavity mode, thus the stability is greatly improved. As only SMF is used in the cavity, the operating wavelength can be tunable without restriction from the self-lasing cavity mode. Furthermore, it proves that core-pumped single-frequency fiber laser is able to generate watt-level power. The laser demonstrates excellent performance in terms of noise, linewidth, and stability.