Laser absorption spectroscopy is widely used for monitoring trace gases in the atmosphere. However, some gases have several overlapping absorption lines. Therefore, the peak signals overlap; this affects the measured results. For example, the NH₃ peak near 6529 cm ^-1 includes four overlapping absorption lines. Absorption spectra at different pressures are simulated to obtain four absorption lines separated by Voigt-line profiles. First, the experimental platform is established by the direct absorption method. The experiment demonstrates that the peak value of the NH₃ spectra at low gas concentration does not decrease. However, the peak value increases at a lower gas pressure. The phenomenon may be attributed to the absorbability of NH₃. Therefore, the CH₄ gas, which does not exhibit absorbability, is used to verify this assumption. Finally, by analyzing the NH₃ absorption lines at different pressures, it is determined that the difference between the experimental and simulated spectra is the smallest when the gas pressure is 0.18 atm(≈18 kPa)and the maximum absorption peak of NH₃ is proportional to the gas concentration. This result solves the interference problem, which will improve future gas-detection approaches.