1. A training sequence generation method applicable to an optical communications system, the method comprising: generating a pseudo random sequence, wherein a number of pseudo random numbers in the pseudo random sequence is equal to half a number of subcarriers comprised in one symbol in the optical communications system; obtaining a chirp coefficient of a modulator used in the optical communications system; using a negated chirp coefficient to modulate the pseudo random sequence to generate a chirped pseudo random sequence; constructing a training symbol segment that comprises L subcarriers in a frequency domain, wherein L is equal to half the number of subcarriers comprised in the one symbol in the optical communications system, wherein, in the training symbol segment, a signal sent over a subcarrier with an even ordinal number is a frequency domain signal of a pseudo random number with the same ordinal number in the chirped pseudo random sequence, and a signal sent over a subcarrier with an odd ordinal number is a zero-level signal; transforming the training symbol segment from the frequency domain to a time domain to obtain a training symbol segment in the time domain; and generating a training sequence based on the training symbol segment in the time domain and outputting the training sequence, wherein the time domain of the training sequence structurally comprises two same training symbols, and each of the training symbols comprises a cyclic prefix and two training symbol segments in the time domain.