The invention relates to a foundation pit top horizontal displacement monitoring method based on opposite side distance measurement and belongs to the technical field of foundation pit displacement monitoring. The foundation pit top horizontal displacement monitoring method comprises the following steps that S1, a total station is arranged freely at any point P around a foundation pit, and the coordinates P of the free station arranging point P are calculated by observing any two given datum points C1 and C2; S2, after the step S1, a foundation pit monitoring point Wi is observed immediately,the slant distance from the total station to the datum points C1 and C2, the zenith distance and the horizontal circle degree are obtained, and the horizontal distance and azimuth angle from the datumpoint C2 to the monitoring point Wi are calculated; S3, the convergence value of the horizontal distance of the foundation pit monitoring point is calculated; and S4, the component of the horizontaldistance of the foundation pit monitoring point in the slope direction, the horizontal displacement and horizontal displacement cumulative variable quantity are calculated. The foundation pit top horizontal displacement monitoring method can adapt to the complex environment of a construction site very well, application is wider, and measurement precision of the monitoring point can be improved.