A polarization rotator and an optical signal processing method are disclosed. The polarization rotator and the optical signal processing method relate to an optoelectronic product field, and can achieve higher conversion efficiency and ensure a smaller size of the polarization rotator in a process of converting optical signal polarization states. A first transceiving waveguide includes a first end and a second end; a polarization rotation region waveguide includes a first waveguide and a second waveguide, where the first waveguide is located above the second waveguide, the first waveguide is connected to the second end of the first transceiving waveguide, the first waveguide and the second waveguide are non-linear profile waveguides; a mode conversion region waveguide includes a third waveguide and a fourth waveguide, where the third waveguide is connected to the second waveguide, the fourth waveguide is on a same horizontal plane as the third waveguide and the second waveguide, and the third waveguide and the fourth waveguide are non-linear profile waveguides; and the second transceiving waveguide includes a third end and a fourth end, where the third end of the second transceiving waveguide is connected to the fourth waveguide. The method is used for conversion between a transverse electric wave mode optical signal and a transverse magnetic wave mode optical signal.