The invention discloses a long-length base line. The long-length base line comprises a foundation, guide rail platforms and linear guide rails, wherein the plurality of guide rail platforms are spliced along q straight line and are mounted on the foundation through hoisting supports arranged at the bottoms of the guide rail platforms; left and right side walls of the guide rail platforms in a splicing direction are provided with protruding positioning blocks; positioning plates are arranged at the left and right sides on the foundation in the splicing direction of the guide rail platforms; positioning grooves for inserting the positioning blocks are formed in the positioning plates; positioning screw holes are formed in two side walls along the splicing direction of the guide rail platforms, a top side wall in a vertical direction and a transverse wall surface vertical to the splicing direction of the guide rail platforms of each positioning groove; the plurality of linear guide railsare spliced and paved on the top surfaces of the guide rail platforms along the splicing direction of the guide rail platforms. According to the long-length base line disclosed by the invention, a guide rail system is used for replacing a traditional observation pier, so that the measurement efficiency is effectively improved, and automatic measurement in a whole process can be realized; the laborintensity of a measurement process is reduced, and a standard value is measured through an interferometer; the detection and calibration precision is effectively improved.