Abstract: For deep foundation pit supporting structure, the polar coordinate method, small angle method or sight line method are generally used to observe the horizontal displacement, but the work efficiency is not high.In the automatic monitoring for foundation pit displacement, the fixed station total station is usually used, so how to quickly and accurately convert the coordinate value of the monitoring point into the horizontal displacement value is the key problem.To solve this problem, this paper proposed a new automatic horizontal displacement monitoring algorithm for foundation pit.By considering the contour of the outer edge of the foundation pit, this algorithm introduces the virtual center point of the foundation pit as auxiliary calculation, which can quickly and accurately reduce the observed value to the vertical direction of the corresponding edge of the foundation pit, obtain the accurate horizontal displacement on each edge of the foundation pit, and improve the calculation efficiency and accuracy.This algorithm is applied to the actual case of horizontal displacement monitoring of a deep foundation pit, and the monitoring accuracy was verified by using the high precision visual line method and polar coordinate method.The results showed that the deviation rate between the horizontal displacement calculation method proposed in this paper and the visual line method was less than 15%.The method can accurately reflect the actual deformation of the horizontal displacement monitoring point, and improve the internal work efficiency.