A new method of measuring guide straightness error is proposed. Based on a linear relationship between polarizing angle and lateral offset position displacement. A peculiar polarized beam, whose polarization angles are distributed linearly as lateral direction coordinates, is modulated by a designed active optical modulator based on polarization interference principle. The polarizing angles of the beam are detected by an optical detecting component and an optical slit, which move correspondingly with the existed guide straightness error. Laboratory experiments are conducted to validate the method and the results show that the correlation coefficients of the linear fitting curves between the polarizing angle and offset displacement is above 0.9995, with the standard deviation of the repeatability test within 1 μm. The range of straightness error measurement is above 0.5 mm with the resolution level at submicron grade. This method minimizes the effects caused by the variation of light intensity and guide surface error and possesses the advantages of convenience, high reliability and accuracy. After modification, it can measure two-dimensional straightuess error with the same precision as the autocollimator. It can be applied to actual industrial applications.