A novel technique, for the fabrication of large size Yb 3+ doped silica glass rod with a diameter of 3 mm and a length of 270 mm and thus a large-core double-clad fiber with a diameter of 80 μm and an outer cladding layer diameter of 400 μm by the glass phase-separation technology, is reported. The refractive index profile, Yb 3+ absorption spectrum and background loss are experimentally tested, and the laser performances are also demonstrated. The results show that the refractive index distributes uniformly within the active fiber core of this fiber and the the numerical aperture is about 0.065. The doping concentration of Yb 3+ (mass fraction) is 1.22%, the absorption coefficient at 976 nm is 6.5 dB/m, and the background loss at 793 nm is 0.03 dB/m. Based on the power amplifier structure of the master oscillator, the lasing output at 1080 nm is realized for this fiber when pumped by a diode laser at 976 nm, where the fiber length is 2.5 m, the slope efficiency is 78%, and the maximum laser output power is 300 W. The glass phase-separation technique provides a novel technique route to fabricate the active silica glass core rods with large size and high uniformity, which has a high potential in the fabrication of heavily doped extra-large-core fibers and active fibers with a complex core structure.