A measurement method and apparatus for a residual direct-current component and a system where the measurement method for a residual direct-current component includes: selecting a section of data of a first predetermined length from a received signal; performing fast Fourier transform on the selected data to obtain a frequency-domain signal of the data; calculating a power spectrum of the frequency-domain signal of the transformed data; and finding out a maximum peak value of the power spectrum in a frequency offset range, and taking power of the maximum peak value as power of the residual direct-current component. With the embodiments of this disclosure, a residual direct-current component of an optical transmitter may be measured at a receiver end of a coherent optical communication system, thereby avoiding a diagnosis error of a communication network, and improving performance of the communication system.