The monitoring of horizontal displacement field of deep soil is an important index for the safe excavation of foundation pit. The existing inclinometer method consumes a lot of manpower and has a low accuracy, but it is still the main method to obtain horizontal displacement at present. A distributed optical fiber measurement method based on the optical frequency domain reflectometry (OFDR) technology is proposed in this paper. The conversion relationship between the strain-deflection deformations is derived. The feasibility and accuracy of the distributed optical fiber inclination measurement are verified by indoor experiments. The measurement method based on OFDR has been applied to a foundation pit project in Suzhou, and the displacement information of deep soil during the excavation process is obtained accurately.