Methane (CH₄) currently stands as a significant clean energy source, constituting a primary component of natural gas. However, due to its highly flammable and explosive properties, monitoring CH₄ concentrations in the atmosphere and critical locations is paramount. Laser absorption spectroscopy, with its advantages of high sensitivity, rapid detection, excellent selectivity, and non-contact capabilities, has found extensive applications in gas measurements and related fields. Optical multi-pass cells (MPCs) are often employed to increase the optical path length (OPL) to achieve higher measurement accuracy. Real-time and precise calibration of the optical path length is of utmost significance. The concentration of the measured gas can be directly inverted by using Lambert Beer’s law through the accurate value of optical path length and other parameters, avoiding the complex step of standard gas calibration in traditional methods. Due to the complex structure and high computational complexity of methods such as Frequency Modulated Continuous Wave (FMCW) and Optical Frequency Domain Reflectometer (OFDR) proposed by previous researchers, we propose a method for multi-pass cell internal optical path length measurement based on Amplitude Modulated Continuous Wave(AMCW) technology in this study, which has the advantages of simple structure and fast measurement speed. This method is integrated with laser absorption spectroscopy to measure the optical path length and CH₄ absorption spectrum simultaneously. The laser beams, one with a center wavelength of 650 nm for measuring the optical path length and another from a Distributed Feedback (DFB) laser with a center wavelength of 1 654 nm for measuring the absorption spectrum, are simultaneously coupled into a multi-pass cell with a physical base length of 12 cm using fiber couplers. At the exit end, the amplitude modulation phase of the laser for the optical path length measurement and the optical intensity of the laser for CH₄ absorption spectrum measurement are measured to obtain both optical path length and absorption spectrum information simultaneously. Measurements were conducted using a standard CH₄ gas with a volume fraction of 297×10^-6 and absorption lines of CH₄ near 6 057.1 cm^-1. First, the output wavenumber of the DFB laser at different operating currents was calibrated, which allowed the transformation of the absorption spectrum’s x-axis from point numbers to wavenumber. Next, the incident angle of light entering the multi-pass cell was adjusted, and data for 4 sets of different optical path lengths and absorption spectra were measured. The internal optical path lengths of the multi-pass cell and the corresponding absorption peak values were 1.606 m and 0.021 2, 3.326 m and 0.044 5, 5.050 m and 0.067 8, and 6.762 m and 0.089 9, respectively. Linear fitting was applied to the measured optical path lengths and the ones estimated from the number of reflections, yielding a high correlation coefficient r≈1. Additionally, linear fitting was conducted between the measured optical path lengths and the absorption peak values, demonstrating excellent linearity with r≈0.999 87. These results validate the feasibility and accuracy of the AMCW technology for real-time measurement of internal optical path lengths within the multi-pass cell, providing a novel method and approach for determining the optical path length and measuring concentration in laser absorption spectroscopy.