Pre-welding rail inspection is a significant step to ensure the safe operation of railway vehicles. Rail straightness is an important index to measure rail quality. Aiming at the problems of cumbersome traditional rail flatness detection steps, limited single measurement length and low measurement efficiency, according to the triangulation measurement principle, the overlapping of laser profiler is used to obtain the rail contour data, and the improved ICP algorithm is used to quickly register the point cloud data to complete the three-dimensional reconstruction of rail. Then the adaptive median filter is used to optimize the flatness parameter curve, and the analog ruler method is used to solve the rail flatness. The experimental results show that the flatness extraction algorithm based on rail shape reconstruction has the advantages of high speed, high precision and good stability. The maximum measurement error with manual detection is 0. 021 mm, the maximum error with high-precision electronic leveling ruler is 0. 011 mm and the maximum standard deviation is 0. 006 mm, which meet the requirements of flatness detection before rail welding.