The invention discloses a system and a method using a fiber Bragg grating equilibrium RSOA to modulate bandwidth, and relates to the technical field of optical communication. The system comprises an optical line terminal OLT with an optical equalizer. The optical line terminal OLT is connected with a remote node RN through a feeder fiber FF. The RN is connected with an optical network unit ONU through a branch fiber DF. The optical equalizer comprises a fiber Bragg grating FBG and a circulator. The method comprises the steps that the OLT sends a downlink traffic signal and an uplink optical carrier; the downlink traffic signal and the uplink optical carrier enter the remote node RN through the feeder fiber, and are demultiplexed into a downlink traffic signal and an uplink optical carrier in the remote node RN, wherein the downlink traffic signal and the uplink optical carrier come to the same sub optical network unit in the ONU and are sent into the ONU through the branch fiber DF; and the downlink traffic signal and the uplink optical carrier are demultiplexed into a downlink traffic signal and an uplink optical carrier through a wavelength division multiplexer in the ONU, wherein the uplink optical carrier is modulated by the RSOA, uploads an uplink signal, returns to the OLT, and is biased and filtered by the optical equalizer in the OLT; a modulation signal whose modulation bandwidth limited injury is balanced is acquired; and uplink transmission with the rate of 10Gb/s is realized.