Natural ventilation can reduce the concentration of indoor pollutants, including that of biological aerosols. It does this mainly by cross ventilation. However, in closely built-up cities, the shielding effect between buildings will significantly reduce the ventilation effect. Previous studies rarely considered the effects of a building's characteristics on other buildings. This preliminary study takes two buildings and investigates the influence of the position and size of nine different windows on their cross ventilation potential. It focuses on only one direction of incoming flow where the distance between the two buildings is two times the width of the building, first, analyzing the reliability of the computational fluid dynamics(CFD) simulation based on steady Reynolds-Averaged Navier-Stokes equations. The results reveal that the reliability of the computational fluid dynamics simulation in some cases is insufficient and that with a simulation of 20% porosity it is difficult to reproduce the wind pressure on a downstream building by computational fluid dynamics in comparison to 10% or 5% porosity. The different simulation reliability may be caused by the instability of the airflow between the buildings. However, using data from the wind tunnel, we found that the cross ventilation potential of the downstream building decreases with the increase of the window area of the upstream building, which is contrary to general beliefs.