This paper presents a new method of measuring guide straightness error based on a linear relationship between polarized 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 polarization angles of the beam are detected by an optical detecting component and an optical slit, which move correspondingly with the existed guide straightness error. Experiments are conducted to validate the method and the results show that the correlation coefficients R<sup>2</sup> of the linear fitting curves between the polarized angle and offset displacement is above 0.9996 with the standard deviation of the repeatability within 1μm, which coincides with the theoretical analysis. The range of measuring straightness error is above 0.5mm with the resolution level at submicron grade. This method minimizes the effects caused by the variation of light intensity and possesses the advantages of convenience, high reliability and accuracy. It is able to be applied in the actual industrial applications.