Embodiments provide a polarizer and a polarization modulation system. The polarizer includes at least one MMI multi-mode waveguide, where one side of each MMI multi-mode waveguide is connected to an input waveguide, and the other side is connected to an output waveguide. An end portion of the side, on which the output waveguide is located, of the MMI multi-mode waveguide is provided with an adjustable portion, and the adjustable portion is connected to the output waveguide. The polarizer further includes a controller connected to the adjustable portion, where the controller is configured to perform control to change a material property of the adjustable portion, so that the output waveguide outputs optical signals in different polarization states.