Abstract: The fourth generation synchrotron radiation light source puts forward higher requirement for the alignment accuracy of the equipment, including the positioning accuracy of the equipment in a large space and the alignment monitoring accuracy of the equipment position after the machine is running. According to the alignment requirement of Hefei Advanced Light Source, the horizontal positioning accuracy within 30 m straight-line section is required to be better than 50 μm. In order to ensure the normal operation of the accelerator, new technical means must be used to monitor the lateral displacement of key components in the accelerator with high precision. The on-line alignment monitoring measurement is an important means to monitor the stability of particle accelerator system. Laser alignment with the characteristics of fast response and high accuracy, is an important data reference source for on-line alignment or adjustment. In this paper, a high-precision long-distance micro displacement monitoring system based on the similarity measure of laser spot was proposed. The lateral position offset of the measured system was monitored by measuring the displacement of laser imaging spot. Firstly, the structure and working principle of the laser light path control and detector assembly of the system were introduced, and the key errors of the system were analyzed, and then the similarity measurement algorithm of the light spot was used to monitor the lateral displacement. The similarity measurement algorithm used in this paper is an image processing algorithm that analyzes the similarity between the template and the search image. The similarity measurement algorithm takes advantage of the stability characteristics of the laser spot and uses interpolation function to calculate the minimum difference between the template and the search image, so as to obtain the position change value of the spot in the search image. Under the influence of random noise, the accuracy of the algorithm is better than ±0.17 pixels (pix). Finally, a 40 m long prototype system was built. Using the similarity measurement algorithm, the relative position deviation of the measured targets at different monitoring positions was obtained at different times. After testing, the horizontal monitoring accuracy of the system is better than 8 μm and the monitoring accuracy of the prototype is not related to the distance of the measured target. The successful development of the prototype system can provide basic data for the development of further micro displacement detection system, and can be applied to the on-line displacement monitoring of accelerator equipment such as fourth generation synchrotron radiation light sources, free electron lasers, linear colliders, etc.