Abstract: In order to realize the direct and continuous measurement of tritium concentration in air at low levels, it is necessary to study the effect of air on the measurement after its direct introduction into the proportional counter. In this study, physical parameters such as the first Townsend coefficient, electron attachment coefficient, diffusion coefficient, and drift velocity of gases with different air ratios were simulated by Garfield++ software, and the effects of air on the plane curve, energy spectrum, and detection efficiency of the proportional counter were experimentally investigated. The results show that the first Townsend coefficient decreases with increasing air ratio, the electron attachment coefficient increases with increasing air ratio, the drift velocity increases with increasing air ratio, the transverse diffusion coefficient decreases with increasing air ratio, and the air negatively affects the measurement of the proportional counter. Air makes the plane curve of the proportional counter change, when the air content increases from 0 to 5%, the plane length is shortened from 260 V to 160 V; when the air content is more than 10%, there is no plane area in the plane curve. The air causes the detection efficiency of the detector to decrease; when the air content increases from 0 to 10%, the detection efficiency decreases rapidly from about 80% to 12%; when the air content is greater than 25%, the detection efficiency is only about 3%; the higher the air content, the lower the detection efficiency. Under the voltage of 1 980 V, the energy spectrum becomes narrower with the air content, and the peak of the energy spectrum basically disappears when the air content is 15%; when the air content is 10%, the increase in voltage makes the energy spectrum broader and reduces the influence of the negatively electrified gas oxygen in the air. The results of this study can provide reference data for the direct and continuous measurement of low-level tritium concentration in air.