To improve ventilation and efficiency in long tunnels under construction, inclined shafts are widely used. This study explores the fire smoke spread behavior in a tunnel under construction with an inclined shaft as the only external connection. Four full-scale experiments varying fire locations reveal the special smoke distribution in tunnels under construction: (1) Fire location alters smoke distribution—main tunnel fires produce symmetric temperatures with limited smoke entering the shaft, while fires in inclined shaft cause rapid smoke spread away from the main tunnel. (2) The ceiling temperature rise follows a negative exponential decay; the decay coefficient k is the smallest (0.0042) for main tunnel fires with low airflow and the largest (0.015) for shaft fires due to the stack effect and strong longitudinal flow. (3) The smoke spread rate in the main tunnel is nearly constant in all cases in the main tunnel but significantly lower for shaft fires. The results show that even a small shaft with limited bifurcation induces airflow that strongly influences smoke spread. The study provides quantitative data for fire risk assessment and smoke control in tunnels under construction.