Abstract: With the rapid development of subway construction, the requirements for safety monitoring of subway tunnels are more and more strict, such as short available monitoring “window time” and high monitoring accuracy requirements. Traditional subway tunnel measuring method cannot monitor subway completely, accurately and rapidly. Therefore, this paper develops a mobile 3D laser scanning system integrating hardware and software to realize millimeter-level high-precision measurement of tunnel structure and tunnel loop-by-loop diameter convergence monitoring, limit detection, ellipticity analysis, staggered analysis, damage detection and so on. In addition, it realizes the tunnel structure monitoring from the traditional geometric surface measurement to accurate identification of its safety state. Compared with the measuring results by traditional total station cross-section measurement and laser range finder direct measurement method, the mobile 3D laser scanning measurement result is reliable. The proposed method has advantages of outstanding real-time, non-contact, digitization, automation, high density, high precision, high efficiency, which can effectively solve the problems in the safety maintenance monitoring of rail transit operations.