Abstract: In order to detect the long rail straightness, laser was used as the reference line, and 2-D position sensitive detector was used as photoelectric converter. When 2-D position sensitive detector was fixed on the guide rail slider and moved along the guide rail, the spot position data was transmitted to the terminal through bluetooth module. After entering the location information, the curve of 2-D straightness was drawn by the software automatically. The results show that, through theoretical analysis, the error of laser deflection angle is calculated and is less than 1μm. The system accuracy is verified by high-precision displacement platform experiment and reaches 3.4μm. In practical appliaction, a long rail of 7.2m is detected and the straightness data of long guide rail is obtained. The repeatability precision can reach 5μm. The study is helpful for straightness measurement of long rails.