Xinchang Tunnel is an extralong highway tunnel with a oneway slope. The air volume required for the left and right lines is significantly different. Aiming at the situation that the ventilation load on the right line of Xinchang Tunnel is too large, a twintube complementary ventilation scheme is proposed, that is, a lateral ventilation duct is added to form a network ventilation system for the left and right tunnels, and the polluted air in the right line is diluted by the fresh air in the left line, so that the air quality both in the left and right tunnels can meet the standard. According to the situation of Xinchang Tunnel project, the feasibility and rationality of the twintube complementary ventilation scheme are studied through scheme comparison. Finally, the twintube complementary ventilation scheme of Xinchang Tunnel is designed. The results show that： By adopting the twintube complementary ventilation scheme for Xinchang Tunnel, it can eliminate facilities installation in inclined shaft and underground fan room on the premise of ensuring the ventilation requirement, and realize good ventilation with less expenses for initial investment and later operation and management. The improved ventilation system enhances driving comfort and safety with significant social and economic benefits.