We present an ultra-wideband and flat gain bismuth (Bi)-doped fiber amplifier (BDFA) in the 1310-1485 nm. Bi-doped silica fiber (BDSF) was fabricated using atomic layer deposition (ALD) combined with modified chemical vapor deposition (MCVD) technique. The absorption and emission characteristics of BDSF were investigated, which has balanced bismuth active centers (BACs) in the O and E bands. Using 95 m BDSF at the 1275 nm single-pump double-pass configuration, a gain of >23 dB was achieved from 1315 nm to 1483 nm (168 nm) with −23 dBm signal power. Furthermore, the −3 dB gain bandwidth was achieved at 106 nm (1342-1448 nm) with a peak gain of 44.0 dB. In addition, we also studied the variation of the gain and noise figure (NF) characteristics of BDFA with the signal power. The ultra-wideband and flat gain BDFA contributes to efficient signal amplification and transmission in communication systems.