<p>The development and utilization of underground spaces effectively alleviate social problems such as energy scarcity and environmental pollution. However, the harmful gases produced during construction period of underground spaces posed a serious impact on the health of workers. To efficiently discharge the harmful gases and improve the construction efficiency, a combination of field measurement and numerical simulation was conducted on the spatiotemporal diffusion characteristics of the harmful gases and the safe re-entry time in a T-shaped blasting tunnel with short straight section. The field measurement results indicated that CO was the main target for harmful gas prevention and control. The numerical simulation results showed that the harmful gases produced by the short blasting metro tunnel posed pollution to the non-blasting metro tunnel, and the distribution characteristics of the supply air volume were significantly different from those in long straight tunnels. Furthermore, the relationship between CO diffusion distance in construction channel and ventilation time exhibited a quadratic polynomial function. While the safe re-entry time presented a power function relationship with the average cross-sectional air velocity. The increase of supply air volume can shorten the safe re-entry time, however the shortening effect gradually weakened, accompanied by an increase in ventilation energy consumption. The research can provide valuable insights into the healthy and efficient control of underground construction environments.</p>