To understand the effect of tunnel construction in a karst ridge-trough area (Zhongliang Mountain, Beibei District, Chongqing) on the characteristics of the soil CO₂ concentration, we analyzed the soil CO₂ concentration and environmental factors of Fraxinus chinensis (FC) at 0-20 cm and 20-40 cm in tunnel affected and non-tunnel affected areas from December 1, 2017 to November 25, 2018. In addition, the soil CO₂ concentration and its related environmental factors under cultivated land (CU), shrub (SH), and bamboo forest (BA) were studied from March 22, 2017 to January 18, 2018 to discuss the variation of the soil CO₂ concentration in karst areas affected and not affected by the tunnel. The results showed that (1) the average values for the soil CO₂ concentrations of A-CU, A-SH, A-BA, and A-FC in area A (tunnel affected area) were 4479.26 mg/m³, 6053.10 mg/m³, 8152.70 mg/m³, and 17162.47 mg/m³, respectively, and those of B-CU, B-SH, B-BA, and B-FC in area B (non-tunnel affected area) were 6244.67 mg/m³, 6647.01 mg/m³, 9422.94 mg/m³, and 18396.09 mg/m³, respectively. The soil CO₂ concentration in the tunnel affected area was lower than that in the non-tunnel affected area. (2) However, the soil CO₂ concentration in the tunnel affected and non-tunnel affected areas had the same vertical and seasonal variation trends:in the vertical change, the soil CO₂ concentration increased with increase in soil depth, and in the seasonal change, the soil CO₂ concentration in the rainy season (summer and autumn) was greater than that in the dry season (winter and spring). (3) Correlation analysis showed that the soil temperature was the main controlling factor affecting the soil CO₂ concentration. Additionally, the soil CO₂ concentration increased with the soil temperature, except during the rainy season, where the soil moisture inhibited the production of CO₂ in the soil. Moreover, the soil physical and chemical properties could also affect the soil CO₂ concentration. (4) Finally, the changes in the soil CO₂ concentration in the tunnel affected areas were more susceptible to changes in the external environment.