A method and an apparatus for determining an optical signal-to-noise ratio are provided. The method includes: obtaining a first optical signal on a to-be-measured channel; converting the first optical signal to an electrical signal and extracting different frequency components of the electrical signal, where roll-off factors of the different frequency components of the electrical signal are the same as roll-off factors of corresponding frequency components of the first optical signal, and the roll-off factor is a ratio of a signal power of a non-center frequency component to a signal power of a center frequency component on the channel; measuring powers of the different frequency components; and determining an optical signal-to-noise ratio of the to-be-measured channel according to the powers and the roll-off factors of the different frequency components. With the method, an optical signal-to-noise ratio of a to-be-measured channel can be highly accurately determined.