Abstract: The Huangjialiang tunnel on Xi′ an-Chengdu passenger dedicated line is located in the front structural belt of northern Longmenshan thrust, containing multiple sets of Cambrian-Triassic source rocks and oil-gas reservoirs, as well as multiple oil-bearing reservoirs, e.g. Tianjingshan and Kuangshanliang, so that the geological background for oil and gas generation is desirable. In this paper, the components of crude oil and harmful gases in the Huangjialiang tunnel are determined through the on-site inspection and indoor test. The results show that the crude oil mainly contains carbon element, followed by hydrogen element. Hazardous gases are mainly hydrocarbon gases,of which methane content is the most, and the non-hydrocarbon gases are mainly H2S, CO2 and N2. The determination of carbon isotope of oil and gas samples indicates that both crude oil and harmful gases come from the Lower Cambrian black mudstone. In combination with engineering situation of the tunnel, it could be seen that crude oil might pollute groundwater and soil vegetation and also corrode the supporting structure. When reaching a certain concentration, the harmful gas might explode and also threaten the life and safety of constructors. Therefore, in the actual construction process, appropriate treatment measures should be taken to reduce the impact of oil and gas on construction progress and safety.