The emergence of harmful gas will bring serious difficulties to the tunnel construction, improper handling can easily lead to accidents. In order to evaluate the possibility of harmful gas in the tunnel, the basic characteristics of harmful gas in tunnel are analyzed from three aspects, that is spatial distribution, inflammable and explosive property, and harm to human body. Based on domestic and foreign research data, the case of harmful gas is listed, and the potential harmful gas is discussed. According to the engineering case, the lithology and geological structure of harmful gas is analyzed. Harmful gases are mainly distributed in the semi hard rock and organic matter rock formations, harmful gas is also prone to gather in the deep rock mass fracture zone, the joint and fracture zone. In order to deal with the harmful gas, we should strengthen the prediction and monitoring, and early detection. At the same time, take ventilation, pressure, grouting and other measures to deal with the harmful gases. The study could provide an effective reference for the similar projects. This research has some practical value and social significance.