Abstract: There are two characteristics of free station method.Firstly it can overcome the restriction of visual condition in complex construction situation;secondly it is easy to obtain high observation accuracy.It is widely used in the deformation monitoring of horizontal displacement of deep foundation pits in cities.According to the principle of survey adjustment,the affecting factors of the measurement accuracy of free station method are deduced,and the influencing law of various factors on measurement accuracy is proposed.The application conditions of satisfying different accuracy requirements in the horizontal displacement monitoring of foundation pits are also summarized.The conclusions are of practical significance for the deformation monitoring of foundation pits and other similar projects.