Disclosed are a polarization rotator and an optical signal processing method, which relate to the field of optoelectronic products, and can both realize higher conversion efficiency and guarantee a smaller volume of a polarization rotator in the process of converting a polarization state of an optical signal. A first transceiving waveguide comprises a first end and a second end; a polarization rotation area waveguide comprises a first waveguide and a second waveguide, wherein the first waveguide is located above the second waveguide, the first waveguide is connected to the second end of the first transceiving waveguide, and the first waveguide and the second waveguide are non-linear profile waveguides; a mode conversion area waveguide comprises a third waveguide and a fourth waveguide, wherein the third waveguide is connected to the second waveguide, the fourth waveguide is in the 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 a second transceiving waveguide comprises a third end and a fourth end, wherein the third end of the second transceiving waveguide is connected tothe fourth waveguide. The method is used for conversion between a transverse electric wave mode optical signal and a transverse magnetic wave mode optical signal.