It is of great significance to clarify the influence mechanism of key factors such as high altitude on CO migration under variable air volume ventilation in construction tunnels and accurately determine the concentration distribution of CO after tunnel face blasting. In this study, a case study is conducted on a construction tunnel with press-in ventilation at an altitude of 3 500 m, and a three-dimensional unsteady ventilation numerical model based on field measurement data is constructed. Professional numerical simulation software is utilized to analyze the dynamic migration and spatiotemporal evolution characteristics of CO under different working conditions. Further, the grey relation analysis method is employed to quantitatively analyze the correlation degree between the time required for CO concentration to decrease to the exposure limit and each influencing factor. The results show the following: (1) During the ventilation process after blasting, the migration distance of the CO peak value section in the tunnel satisfies a quadratic function relationship with ventilation time. (2) By comparing the spatiotemporal distribution characteristics of CO in the tunnel under different working conditions, it is found that the lower the altitude, the smaller the amount of explosives used, the closer the end of the air duct to the tunnel face, and the higher the hanging position of the air duct, the shorter the 〖JP2〗ventilation time required for the CO concentration to decrease to the exposure limit. (3) The grey relation analysis results show that among the influencing factors, the amount of explosives and the distance from the end of the air duct to the tunnel face exhibit the most remarkable influence on the time required for CO concentration to drop to the exposure limit, with grey relation degree coefficients of 0.77 and 0.76, respectively; the altitude comes next with a correlation coefficient of 0.68; and the influence of the air duct hanging position is relatively limited, with a correlation coefficient of 0.49.