Underground streets have been developed in cities with land shortages. We simulated the dynamic distribution characteristics of smoke flow and temperature from fires under natural conditions in underground streets. The experiment was carried out at the Sichuan Fire Research Institute of Public Security Ministry, P.R. China. A 500 kg fire source was used in a real underground street. Temperature distribution at different heights and personnel escape routes are shown in two typical sections. The comparison between the computer simulation data and experimental data validates the simulation. This study provides a valuable reference for fire assessment of underground shopping streets.