As an important parameter of the coherent optical system,the linewidth of narrow fiber laser must be measured accurately. Compared with other currently available linewidth measurement techniques, the Delayed Self-heterodyne Interferometer (DSHI) technique is the most promising one. In this paper, theory of the DSHI for determining the linewidth of lasers is described in detail. The MATLAB emulator is programmed to analyze the effects of fiber delay line on the results. Furthermore, an experimental setup based on the DSHI is constructed to measure IPG’s fiber laser linewidth. We substitute oscilloscope for the spectrum analyzer in this system, and the software algorithm is designed for the analysis of the photocurrent signal. From numerical analysis, the measured linewidth of the IPG’s fiber laser is about 16kHz, which is in the range of an ideal accuracy.