A polarization-independent Delayed Self-Heterodyne Interferometer（DSHI） is introduced

the optical system of which is a Faraday Rotator Mirror（FRM）-based Michelson interferometer

and its laser linewidth measurement principle and polarization-independent characteristics theoretically analyzed.On this basis

the polarization state stability measurement for the traditional Mach-Zehnder（M-Z） DSHI and the polarization-independent Michelson DSHI are performed respectively and compared with that for the ultra-narrow（4.5 kHz） laser linewidth.The results indicate that the Michelson DSHI with high polarization state stability has a higher accuracy in laser linewidth measurement.Furthermore

by using this type of DSHI

the linewidth measurement for another ultra-narrow semiconductor laser with a linewith of 1.7 kHz is implemented in an ideal range of accuracy.