Abstract: Laser collimation is increasingly applied to dam deformation monitoring, especially vacuum laser is on favor. However, under a certain condition air laser can do it completely, and the reliability and accuracy of observation is good. Air laser collimation sys-tem has been used to observe horizontal displacement of dam in Liujiaxia Hydropower Station since early1970s and have been working well.Up to now, it realized observation automation instead of artificial observation by much improvement and got a good result.