In order to solve problems in linewidth measurement for narrow linewidth laser, a method is put forward based on the phase noise of unbalanced fiber-optic interferometer. The relationship between the phase noise of an unbalanced fiber-optic interferometer with a short optical path difference (OPD) and the laser frequency jitter is analyzed, and the optical power spectrum and linewidth are obtained. The linewidth of a distributed feedback laser is tested using an interferometer with 10 m OPD. The result shows that the power spectrum is an approximate Lorentzian lineshape with 5.4 kHz linewidth, which is close to the theoretical value of the laser. The linewidth of a fiber ring laser (FRL) is tested and the result is 0.75 kHz, which is more accurate than the result of less than 1.5 kHz of the similar FRL obtained using homodyne method.