CFD simulation is a useful tool for studying. However, in reality there are often complex, unsteady air flow patterns and large geometry domain and complex boundary conditions which are very difficult to totally take into consideration in the simulations. So sometimes we made the calculation domain not the same with geometry domain and simplified the boundary conditions. In this paper, five cases were made to study the calculation domain and boundary conditions for push-pull ventilation. According to the analyses and calculations the walls with windows and door closed setting for wall boundary conditions were not correct. On that basis cracks added, and the boundary conditions were pressure-inlet, and the pressure was zero. The calculation domain was reduced, the result was some different: the tendency was the same, but the difference of specific point values was some big. The new boundaries of the reduce calculation domain were set for pressure-inlet, and the pressure was zero. Under this condition, the cracks could be simplified to wall boundary conditions.