Laser-interferometric measurements, possessing the ultra-high sensitivity, have been widely applied to fundamental scientific research and practical technological developments. However, low-frequency classic noise (less than 1 Hz) dominates exclusively in the interferometer system and measurement of sub-Hertz physical signals beyond the shot noise limit is still facing great challenges. Using the degree of freedom of laser polarization as the separation channel of light energy in the interferometer and the sub-Hertz low-frequency physical signal as the detection target, the key problem of difficult acquisition of local oscillator light in the readout scheme of the laser interference signal is solved. A dual-frequency working laser is used to convert the low-frequency displacement signal into high-frequency AC photoelectric signal at the system output to naturally avoid the technical problems of 1-Hz band electronic noise, thereby providing important theoretical guidance and experimental support for quantum noise limit ultra-low-frequency laser interference and interference signal readout method. It is expected to provide a reference path and scheme for upgrading ground gravitational wave antenna.