The linewidth measurement error of delayed self-heterodyne interferometer is described in detail. The fundamental using delayed self-heterodyne to measure laser linewidth is analyzed, the emulator is programmed and an experimental setup is constructed to measure laser linewidth, when shorter delayed fiber is adopted. The result shows the measurement error result from the fluctuation of power term. From numerical analysis, the power spectrum resulting of self-heterodyne signal is Voigt profile and the 1/f frequency noise is shown to be approximately Gaussian. The effects of 1/f frequency noise on self-heterodyne detection are described and the results are applied to the problem of laser linewidth measurement, It is proved that method is an effective tool for reducing the measurement error. As the example, when the spectrum width of Gauss profile is 4.5 kHz, the Lorentzian spectrum width is 0.68 kHz, the improvement is abvious.