The newly sensitivity enhancement mechanism about Au & black phosphorus composite film based on surface plasma resonance (SPR) fiber sensor. The sensor was fabricated by a 35 nm aurum (Au) film and 5–10 nm black phosphorus (BP) thin film, which integrated to the surface of fiber. The sensitivity enhancement level can reach as high as 140 % compared with the common Au SPR sensor at the 1.35 refractive index (RI) region. In addition, this newly style fiber structure is extremely suitable for the biochemical sensing.