For large complex electromechanical systems such as aeronautics and astronautics, the demand for length measurement has also evolved from one-dimensional displacement measurement to multi-objective, flexible and extensible multiline parallel measurement. The frequency-modulated continuous-wave lidar length measurement technology has the advantages of anti-interference, high precision and no cooperation. In this paper, a multiline parallel coherent precision length measurement method based on frequency modulation continuous wave laser lidar is studied. To solve the problem of fiber dispersion and auxiliary fiber drift in the traditional length measurement system, a three-channel Mach-Zehnder interferometry system containing a hydrogen cyanide gas absorption cell is proposed. On this basis, the precision length multiline parallel measurement is realized based on the optical switch structure. The experimental results of precision guide rail length measurement show that the proposed method is less than 25 μm in the length range of 3.6 m. The four-way parallel length measurement is realized. The absolute error of the multi-channel measurement results is not more than 30 μm compared with the measurement results of a commercial laser interferometer.