To explore CO diffusion patterns under duct leakage during press-in ventilation in drill-and-blast tunnels, a case study is conducted using computational fluid dynamics simulations with the κ-ε turbulence model. Leakage hole area, number, and position are taken as key variables to simulate and analyze the airflow and CO distribution under various leakage scenarios after blasting. Results show that: (1) Larger leakage areas intensify vortex-induced backflow, while decreased airflow energy and pressure reduce leakage near the tunnel face. (2) Leakage inhibits CO accumulation at the tunnel ceiling but causes local CO buildup near leakage points. (3) Increased area and number of leakage holes suppress upward CO diffusion and slow its spread. (4) Upward-shifted holes divert airflow sideways, pushing CO toward the tunnel shoulders and delaying clearance at breathing height. (5) A multivariate quadratic regression model is developed to correct ventilation time under leakage conditions, providing a theoretical basis for ventilation adjustment.