An updating apparatus (1000, 1100) for an equalizer coefficient, comprising: a signal receiver (1001, 1101), configured to receive an optical signal transmitted by a transmitter in an optical communication system; and a signal processor (1002, 1102), configured to perform coherent detection and analog-to-digital conversion for the optical signal, so as to obtain a digital electric signal; the optical signal comprising transmission data and a constant modulus signal, wherein the constant modulus signal is periodically or aperiodically inserted into the transmission data by a transmitter for updating a coefficient of an equalization filter, the updating apparatus further comprising: an information determiner, configured to determine whether a symbol in the digital electric signal is a symbol corresponding to the constant modulus signal according to symbol timing, or a numerical calculator, configured to detect an output signal of the equalization filter, so as to calculate an average value and/or variance of moduli of the signal and an information determiner, configured to determine whether a symbol in the digital electric signal is a symbol corresponding to the constant modulus signal according to the average value and/or variance of moduli of an output signal of the equalization filter; and a coefficient updater (1003, 1103), configured to update the coefficient of the equalization filter at the symbol corresponding to the constant modulus signal in the digital electric signal. a signal receiver (1001, 1101), configured to receive an optical signal transmitted by a transmitter in an optical communication system; and a signal processor (1002, 1102), configured to perform coherent detection and analog-to-digital conversion for the optical signal, so as to obtain a digital electric signal; the optical signal comprising transmission data and a constant modulus signal, wherein the constant modulus signal is periodically or aperiodically inserted into the transmission data by a transmitter for updating a coefficient of an equalization filter, the updating apparatus further comprising: an information determiner, configured to determine whether a symbol in the digital electric signal is a symbol corresponding to the constant modulus signal according to symbol timing, or a numerical calculator, configured to detect an output signal of the equalization filter, so as to calculate an average value and/or variance of moduli of the signal and an information determiner, configured to determine whether a symbol in the digital electric signal is a symbol corresponding to the constant modulus signal according to the average value and/or variance of moduli of an output signal of the equalization filter; and a coefficient updater (1003, 1103), configured to update the coefficient of the equalization filter at the symbol corresponding to the constant modulus signal in the digital electric signal. an information determiner, configured to determine whether a symbol in the digital electric signal is a symbol corresponding to the constant modulus signal according to symbol timing, or a numerical calculator, configured to detect an output signal of the equalization filter, so as to calculate an average value and/or variance of moduli of the signal and an information determiner, configured to determine whether a symbol in the digital electric signal is a symbol corresponding to the constant modulus signal according to the average value and/or variance of moduli of an output signal of the equalization filter; and a coefficient updater (1003, 1103), configured to update the coefficient of the equalization filter at the symbol corresponding to the constant modulus signal in the digital electric signal.