Embodiments of the present disclosure provide example frequency domain equalization methods, example equalizers, example optical receivers, and example systems. One example method includes obtaining, by an optical receiver, a first complex signal. The first complex signal is a first time domain signal. The first complex signal is obtained based on two channels of mutually independent digital electrical signals. The optical receiver converts the first complex signal into a frequency domain signal, and multiplies the first complex signal in frequency domain by a tap coefficient to obtain a second complex signal. The tap coefficient is used to implement signal compensation for the first complex signal in frequency domain. The optical receiver converts the second complex signal into a second time domain signal, divides the second time domain signal into two channels of real signals, and outputs the two channels of the real signals.