A dynamic dispersion detecting method and apparatus are disclosed. The apparatus includes a tunable dispersion compensation module (101), a demodulator (102), a receiver (103), a partial band radio frequency power detecting unit (104), and an electrical signal ratio calculating unit (105). The method includes: demodulating a phase of a received optical signal; converting the demodulated optical signal into an electrical signal; sampling radio frequency power of the electrical signal to obtain an radio frequency signal; obtaining an electrical signal ratio of the radio frequency signal; and comparing a value of a currently detected electrical signal ratio with the values of the previously detected electrical signal ratios, tuning a dispersion compensation value according to a comparison result to find a peak electrical signal ratio, and obtaining a residual dispersion value of a system according to the peak electrical signal ratio.