To investigate the variation in indoor carbon dioxide (CO₂) volume fractions and their relationship with human activities, this study designs an open-path tunable diode laser absorption spectroscopy (TDLAS) sensing system to monitor indoor CO₂ volume fractions. A distributed feedback (DFB) laser with a central wavelength of 2004 nm is employed as the excitation light source to measure the R(16) characteristic absorption line of carbon dioxide. The Levenberg-Marquardt method of nonlinear least squares fitting is employed to fit the measured spectra, allowing for volume fraction measurements without requiring calibration. Comparative measurements with a commercial XENSIV^TM PAS CO₂ sensor yield a high correlation (R²=0.89). The results indicate that the daily average indoor CO₂ volume fraction is 4.63×10^-4, slightly surpassing outdoor levels, whereas the fluctuation range of indoor CO₂ volume fraction within a day is 3.86×10^-4?5.66×10^-4. Indoor CO₂ volume fraction is volume fractions influenced by ventilation and indoor human activities, and the daily volume fraction trends are highly correlated with working hours. At a personnel density of 0.005 persons/m³, the growth rate of CO₂ volume fraction is measured at 2.3×10^-5 h^-1. Therefore, timely ventilation is recommended for crowded indoor environments to prevent elevated CO₂ volume fractions that may cause discomfort.