To investigate the effect of a solar chimney on the ventilation in a underground space, a dimensionless model was established in this work to numerically analyze internal airflow characteristics and outlet flow regularity of the solar chimney under different Rayleigh number (Ra). A structure optimization measure was proposed to increase the outlet mass flow rate of the chimney. The results indicated that with the increase of Ra, the airflow in the chimney changed from the state of a full outflow to a reverse flow, which occured at the outlet of the chimney when Ra was approximately equal to 10⁶. It was also found that with the increase of Ra, the dimensionless mass flow rate at the outlet increased. But its growth rate decreased when the reverse flow emerged, implying that the reverse flow was one of the important factors affecting the ventilation effect of the underground space. Accordingly, a tilted optimal ventilation structure was put forward.