This paper develops a new detection system, for the first time, to detect the gas concentrations of the SF6/N2 mixture in extra/ultra-high voltage power transmission systems. The concentrations of SF6 and N2 are calculated from the thermal conductivity function of the mixture gas. The main contribution of this work is that a specially-designed thermostatic chamber with adaptive temperature controller is developed to ensure constant pressure of the gas flowed through the thermal conductivity sensor. Another contribution is the combination of multiple sensors (e.g. humidity and electrochemical sensors), which enables the detector to address the penetration effects of H2O and O2 in the SF6/N2 mixture. Experimental evaluation results using the prototype demonstrated that satisfactory accuracy (±1% of the measurement error) has been achieved for the concentration detection of the SF6/N2 mixture under variable operation conditions. Compared with existing detection techniques, the proposed detector not only can detect the SF6/N2 concentration by taking the air infiltration effect into account, but also reduce the cost.