An apparatus for detecting wavelength drift and a system and method for locking wavelengths are disclosed herein so that multiple optical modules can share a wavelength locking apparatus without any wavelength selecting module. The output signals of only one optical module are scrambled at a time, and the identifier information carried in the optical signals output by different optical modules is the same, for example, the same frequency is applied to scrambling of every optical signal. Therefore, the scrambling apparatus is simplified, and the interference between scrambling signals is eliminated. Because no mechanical or electrical apparatus is used for controlling the wavelength switching, the cost is reduced, the locking speed is increased, the accuracy of signal processing is improved, and the precision of wavelength locking is enhanced.