Abstract: In order to develop the measurement technology of small angle and straightness errors, a method of straightness error measurement based on light polarization was proposed. The relation of angle quantity computed by laser speck lateral shift was theoretically derived. Virtual simulation system was constructed, and the feasibility of the proposed method was investigated. Based on the virtual simulation and software modeling, the experimental verification was conducted on a guide rail with the length of 1 m, good linear characteristics and the expected geometric accuracy requirements. The method was compared with autocollimator method. The results show that, the proposed method is in good agreement with the tangent value of small angle measured by the autocollimator in the range of ±1.5×10⁻⁴. The study is helpful for the measurement of small angle and straightness error.