In the dual-path FMCW laser ranging system, the length calibration accuracy of the reference optical fiber directly affects the ranging accuracy. In order to improve the ranging accuracy, a high precision reference optical fiber calibration method was proposed, which was based on hydrogen cyanide(H¹³C¹⁴N) gas cells signal splicing. In this paper, the principle of calibration method based on H¹³C¹⁴N gas cells was deeply researched. Moreover, to reduce the burden of data acquisition, H¹³C¹⁴N signal splicing was used to optimize the calibration method. The experiments show that the calibration method based on H¹³C¹⁴N signal splicing is more stable than that based on laser interferometer traditionally. After calibrating the reference optical fiber by using H¹³C¹⁴N. At the same time, within a measuring range of 3.8 m, the error between ranging value of FMCW system and measuring value of interferometer is less than 14 μm, and the standard deviation is less than 17 μm.