Embodiments of the present invention provide a method, an apparatus, and a system for processing an optical network signal. The method includes: receiving an optical signal sent by an optical line terminal, where the optical signal includes two polarization components perpendicular to each other, and downlink data is modulated on one of the polarization components; dividing the optical signal into two signals, where each signal is the optical signal; demodulating the downlink data from one optical signal and performing, for the other optical signal, vertical polarization rotation processing and processing of modulating uplink data onto two polarization components of the optical signal; sending the other optical signal on which the vertical polarization rotation processing and the uplink data modulation processing are performed to the optical line terminal. With the embodiments of the present invention, signal processing load of the optical network unit and the optical line terminal can be lowered.