A novel kind of dual longitudinal mode He-Ne laser nanometer scale measurement system based on polarization mixture was put forward.With the inserted birefringence element,the frequency splitting effect will occur inside the He-Ne resonant cavity,which makes the laser turn into the dual-frequency laser with adjustable frequency difference.By using frequency splitting effect,mode competition effect and double longitudinal mode power tuning effect of laser physics,a new type of laser nanometer measurement ruler was proposed by using polarization mixing and floating threshold setting.The laser wavelength is applied as a ruler with traceability,leading to the nanometer scale resolution without electrical subdivision.The experimental results demonstrate that this system has the optical resolution of 79 nm,the measuring range of 20 mm,the linearity of 5.8×10-5 and standard deviation of 520 nm.