Tunable diode laser absorption spectroscopy (TDLAS) has been widely used for trace gas detection due to high sensitivity and selectivity. However, measurement results of the optical gas sensors are constantly affected by the pressure of a target gas, especially in atmospheric environment. Most of the existing methods need to install pressure sensors on the spot to correct the measurement results. In this paper, a simple method was proposed for modification of the measured gas concentration without pressure sensor required. A gas absorption line with collision broadening dominant was selected, and wavelength modulation spectroscopy with the first-harmonic (WMS-1f) and the second-harmonic (WMS-2f) detection were performed simultaneously. The analytic expressions characterized by line broadening were established by the spacing between peak and valley of WMS-1f signal and the distance between two zero crossings of WMS-2f signal, respectively. The effect of pressure fluctuation on the measured gas concentration could be corrected by the WMS-1f or 2f signal, due to the relationship between line broadening and gas pressure. Carbon dioxide gas with concentration of 1 980 mg·m-3 was used to verify our method. The CO2 absorption located at 4 989.97 cm-1 was selected for concentration measurement with variable pressures and different modulation depths near atmospheric pressure. The influence of pressure variation on harmonic signals of carbon dioxide absorption spectrum was analyzed experimentally. The gas pressures were inverted by using the WMS-1f peak-valley spacing and the WMS-2f zero-crossing spacing respectively. Compared with the pressure data measured by gas pressure sensor, the pressure deviation was less than 1%. Therefore, the correctness of calculating pressure by analytic expression of harmonic spacing was verified. Finally, the gas concentration corrected by pressure compensation was carried out by using WMS-2f/1f technology. The results showed that the error of the corrected concentration by measuring harmonic spacing is less than 2% compared with that compensated by high-precision pressure gauge, which is consistent with the pressure uncertainty (less than 2%) derived by harmonic spacing. The feasibility and validity of this method are verified. It can further improve the measurement accuracy of TDLAS technology in gas concentration detection under high pressure fluctuation environment. This method is very simple and feasible without requirement of additional gas pressure sensors, which is especially suitable for high sensitivity and high precision open-path measurement of gas composition in atmospheric environment. Moreover, it can also be used for simultaneous measurement of gas concentration and pressure.