The embodiment of the invention discloses a bandwidth allocation method and an optical access network system. The bandwidth allocation method comprises the steps that: a reflective semiconductor optical amplifier of a first optical network unit transmits a REPORT frame to an optical link terminal; the optical link terminal feeds back a generated GATE frame to the first optical network unit; an optical signal with a wavelength of a second optical network unit transmitted by an adjustable transmitter of the first optical network unit is allocated to an output port corresponding to the second optical network unit after passing through an isolator, a coupler and a wavelength selective switch, so as to output the optical signal based on an output channel range corresponding to the output port, and all optical network units in a same group as the second optical network unit receives the optical signals through a respective receiver; and the second optical network unit preserves the optical signal and carries out corresponding processing. By applying the embodiment of the invention, the P2P communication between the optical network units in an optical access network is realized, and the dynamic allocation of bandwidth is achieved.