During the construction of long tunnel, CO gas generated by blasting have a negative impact on construction safety and progress. In order to study the effect of press-in ventilation in long tunnel constructed by the both side drift method, the computational fluid dynamics software Fluent was used to model tunnel ventilation with different heading pits, step lengths and air duct layout positions., analysis the flow field characteristics and CO gas diffusion law were investigated. The results show that the pilot pits and steps in the tunnel will affect the development of jets, the ventilation effect will affected by the vortex areas located in front of the pilot pits and steps. It is easier to form the vortex zone impacting the discharging of CO gas with the increase of the length of the pilot pits and steps, when air duct arrangement conditions is the same. When the tunnel head and step length are the same, and the air duct is arranged on the tunnel vault, the CO gas can be diluted rapidly. This study provides a reference for the ventilation of tunnel using the construction of the both side drift method.