The utility model relates to the field of building security, especially to a building foundation pit slope horizontal displacement monitoring device which comprises a plurality of inclinometer pipes arranged on the periphery of a foundation pit and clinometers for measuring angles of the inclinometer pipes. Each of the inclinometer pipes is composed of a bottom cap, straight tubes and a connecting pipe. The bottom cap is used for sealing a pipe orifice of a straight tube positioned at the innermost end of a drill hole. The connecting pipe is used for connecting adjacent two straight tubes. Guide grooves are arranged along the tube length direction in the inner wall of the straight tube, and two eccentric grooves are arranged in the outer wall along the tube length direction. The inner tube wall of the connecting pipe along the tube length direction is provided with raised strips which match the grooves. Each of the inclinometer pipes is composed of three parts and is convenient for field installation and construction. The guide grooves are used for limiting movement paths of probes of the clinometers and guaranteeing accuracy of detection results. By the two eccentrically-arranged grooves, more accurate butt joint of the straight tubes is guaranteed, and inaccurate measurement of the probe of the clinometers at the joint of two straight tubes is avoided.