The basement space of underground sewage treatment plants is an enclosed flat large-space where workers conduct inspection and maintenance work. Mechanical ventilation system is necessarily installed to ensure indoor air quality. In order to provide theoretical support for scale-model experiments of basement space ventilation, in this paper, several scaled models of the basement space in an underground sewage treatment plant installed with ductless ventilation system are numerically established. The critical Reynolds number, Recrit, to achieve independent flow pattern and pollutant distribution characteristics is investigated mainly with Computational Fluid Dynamic (CFD) method and validated against experimental data. For a 1:40 scaled model, the dimensionless velocity and pollutant concentration field in the basement space are compared when Reynolds number increases from 85 to 12750. The critical Reynolds number is found to be around 6000. Two evaluation indexes, i.e. relative change rate of velocity (RCRV) and deviation rate (DR), are used to evaluate the critical Reynolds number (Recrit). A criterion of evaluation indexes less than 5% is used to quantitatively calculate Recrit. The influence of scale ratio of prototype on Recrit is investigated when it changes from 1:5 to 1:100. The results show that scale ratio has almost no impact on Recrit. It is useful for the design of scale-model experiments for such large space ventilation.