In the context of carbon neutrality, reliable and accurate quantification of atmospheric greenhouse gas is of great importance. Thus, it is necessary to develop gas analyzers which can provide analytes amount-of-substance fraction results traceable to the international system of units (SI). Tunable diode laser absorption spectroscopy (TDLAS) is a common way to measure gas concentration. According to Beer-Lambert Law, one of the necessary conditions to realize the direct traceability of the measured concentration of the instrument to the SI is the directly traceable gas cell optical path length (OPL). The gas cell OPL directly affects the measurement uncertainty of the gas concentration. Accurate measurement is conducive to developing gas analyzers whose measurement results can be directly traced.In this paper, the directly traceable measurement of a gas cell OPL with three reflection points is carried out. The direct measurement result obtained by measuring three parts of the optical path of the gas cell using a calibrated meter is 81.21±0.80 cm. The large measurement uncertainty (0.80 cm) is a careful consideration, estimated considering positioning error and the measurement error caused by the possible misalignment of the three optical paths and the measurement path. In order to reduce the OPL measurement uncertainty, a TDLAS gas cell OPL measurement system is built in this paper. A 1 576 nm distributed feedback laser with a ramp sweep voltage loading on the laser controller is used to measure the absorption spectrum of standard high-purity carbon dioxide (CO₂, 99.999%) in the gas cell to be measured near 6 344.68 cm^-1. A traceable pressure and temperature sensors are used to measure the pressure and gas temperature in the gas cell. The key parameter of line strength of the P 4e branch of CO₂ at 30012-00001 transition band, which has relative uncertainty of 0.15%, is from theNational Institute of Standards and Technology. A quadratic speed-dependent Voigt line profile is used to fitting the acquired absorbance at various pressure from 36 to 75 Torr. The uncertainty and the transfer process of various input parameters are comprehensively analyzed. A generalized linear regression (GLR) is applied to absorbance data with uncertainty at various pressure. The slope from the GLR yield the optical path length of the gas cell L= 81.61±0.42 cm with a standard uncertainty of 0.51%.This uncertainty range falls within one of the direct measurement results. The optical path structure of the gas cell used in this paper is a simplified multi-reflection gas cell with long OPL. The system can be equally applied to the traceable measurement for OPL of the multi-reflection gas cell.