[Purposes] Railway platform gauge constitutes a specialized construction gauge category characterized by the most frequent interaction with pedestrian flow and the narrowest safety margin. In recent years, alongside the rapid advancement of high-speed railway systems, the measurement of curved railway platform gauge demands methodologies with higher precision and efficiency. Therefore, this paper presents a onboard measurement method of railway construction gauge based on laser scanner and inertial navigation system (INS). [Methods] This method can obtain the point cloud data of the platform and the rail, as well as the motion posture data of the detection trolley.Through spatial coordinate transformation, data indexing, extraction of feature points, fitting and alignment of the track centerline, correction of platform point cloud data, and outputting variation curves of platform construction gauges were achieved, thus obtaining the minimum profile through software algorithms. [Findings] This measurement method was used to test the curved platforms of metro lines, and results were compared with those obtained by the manual simplified method. It was found that they were highly consistent, which proved the accuracy of and reliability of this method. [Conclusions] Onboard measurement method of railway construction gauge based on laser scanner and inertial navigation system (INS) can effectively address the profile correction challenge in onboard measurement of railway construction gauge and identify problem of curved platforms.