A new method based on order tracking technique for reducing the effect of nonlinear optical frequency swept in frequency sweeping interferometer(FSI) is presented. The main factors affecting the measuring accuracy of the FSI system are discussed in frequency domain. This method utilizes the optical resonant characteristic of a Fabry-Perot etalon to measure the instantaneous optical frequency of a laser rapidly. Carries on the data linear fitting to the optical frequency on the least- square method and make the slope of the linear equation as the correction rate of optical frequency. The time series of equidistant optical frequency grid can be obtained with polynomial fit and interpolation of optical frequency. The interference signal can be resampled according to time series of equidistant optical frequency grid, and then reconstructed by using data interpolation. The frequency value of non-stationary interference signal is achieved after spectral analysis of reconstructed interference signal. The results indicate that this method dramatically reduces the effects of the nonlinear sweep without limit the range of the measured distance, and demonstrates spatial resolution enhancement of more than 36 times in the range of 1000 mm.