A confined-doped fiber was fabricated by a modified chemical vapor deposition (MCVD) process based on refractive index matching technology. With theory and experiments, we compared the confined-doped fiber and normal-doped fiber. We found that the confined-doped fiber with a core of 35 μm and 0.07 numerical aperture could achieve single-mode output and improve the beam quality from 2.8 to 1.5 in the fiber laser. Meanwhile, it still possesses high laser efficiency and has good stability of beam quality with the increase in pump power. It suggests that the confined-doped fiber with a MCVD process may be the key material for a high-power fiber laser with excellent beam quality.