In high-altitude tunnel blasting operations, reduced atmospheric pressure leads to a rapid increase in carbon monoxide (CO) volume fraction. Designing an optimized layout scheme for CO monitoring sensors is beneficial to improving sensor detection accuracy, shortening suspension period of operations after blasting, mitigating CO exposure risk for construction personnel. A case study is conducted on a pressurized ventilation construction tunnel at an altitude of 3 500 m, and on-site sensor monitoring data and a three-dimensional unsteady ventilation numerical simulation method are employed to analyze the influence of the distance between the ventilation duct outlet and the tunnel face on the flow field characteristics and CO transport patterns. Based on the relationship between the distribution of CO concentration at monitoring sections and the ventilation distance, an optimized sensor placement strategy is proposed. Findings are as follows: (1) In existing construction processes, there is a discrepancy between the monitoring time recorded by deployed sensors and the actual clearance time of CO within the breathing zone height range. This deviation gradually decreases as the distance between the duct outlet and the tunnel face increases. (2) For the tunnel under study, dual sensors should be symmetrically installed on both sidewalls at a height of 3.6 m above the ground when using fixed sensors. (3) The locations of sensors should be dynamically adjusted according to the distance between the end of the ventilation duct and the tunnel face when using movable sensors. When the distance between the end of the ventilation duct and the tunnel face is less than 30 m, priority should be given to installing the sensor on the left sidewall at a height of 3.6 m; whereas the distance is not less than 30 m, it should be installed on the right sidewall at the same height.