The effective and reasonable construction of the low impact development (LID) facilities in loess area depend on the functionality of typical LID facilities and the safety of surrounding structures in areas. A full-scale field test on rainwater concentrated infiltration of bioretentions in a collapsible loess site was conducted in this study. The water content and deformation law of the site were analyzed, and the water movement law of the rainwater-concentrated infiltration at bioretention facilities in the loess site was determined. The site settlements were calculated as per the wetting deformation curve and infiltration depths were calculated on an improved infiltration depth model tailored to the loess area. The rainwater infiltration rules of different bioretention structural forms are different in the collapsible loess field. The diffusion rate of the retaining wall type in loess decreases over time, while that on a sloping type does not. Within the same infiltration time, the retaining wall has a stronger influence on the site than the sloping type. When the water is concentrated in the site, its influence on the subgrade settlement is small (generally less than 1.5 mm) enough to satisfy the relevant engineering requirements. A modified Green-Ampt model based on assumed loess saturated unsaturated stratification can be used to predict the infiltration depth of facility water at the site. The adverse effects of water infiltration related to stagnant bioretentions can be mitigated by adjusting the initial water content and saturated water content at the loess site.