This paper aims to develop a high-precision method for identifying the gas content in the air. Considering the features of ultrasonic wave, an ARM-system was established to detect the content of sulphur hexafluoride, a gas denser than the air. The theories, hardware and software of the system were introduced in details. Then, the system performance was verified through tests on sulphur hexafluoride leakages under the same environment. It is proved that the system can identify the sulphur hexafluoride content in the air with a high precision. With the development of computer, communication and network technologies, the proposed system will have broad prospects in the detection of gas content.