A laser linewidth measurement system based on an unbalanced Michelson interferometer achieved using a 3 × 3 coupler and phase signal demodulation is reported in this study. The signal processing section is based on phase signal demodulation technology， which can process the signal in real time and at high speed. Therefore， the frequency noise and linewidth of the laser under test could be obtained quickly. The system features a simple optical path structure， no active control， and stable repeatability of measurement results. However， it is necessary to consider whether the sampled signal contains the maximum and minimum values of the source signal simultaneously when using this scheme. Therefore， the effect on the frequency noise power spectral density of the laser under test is thoroughly discussed with respect to different sampling windows of 0.5， 0.4， 0.1， 0.05， and 0.01 s. Both simulation and experiment show that the amplitude of the laser frequency noise power spectral density calculated by the signal is too high， and that sampling signals with sampling windows of 0.1， 0.05， and 0.01 s do not contain the maximum and minimum of the source signal. A commercial laser with a wavelength of 1.5 μm and a self-made laser in our laboratory with a wavelength of 2 μm are measured using β-separation line method. The results show that the linewidth of the commercial laser in the 1.5 μm band is 5 kHz， with a measurement time of 2 ms. It also shows that this conclusion can be extended to the entire band.