Bioretention is an important technology for ecological control of runoff. The purpose of this study was to investigate the coupling effect of in-situ soil and groundwater level on the hydrological performance of bioretention. VADOSE/W was used to simulate the water transport processes during bioretention under a single rainfall event. The effects of four in-situ soil types and two groundwater levels on the surface ponding, underdrain outflow, exfiltration, and runoff regulation effects of bioretention were studied. Under eight geological situations and the rainfall of 0.17 mm/h (6.0 h), the ponding duration and overflow volume of bioretention were 556-649 min and 24.71-39.61 mm/m², respectively; the underdrain outflow peak value and duration were 0.549-0.804 mm/min and 380-730 min, respectively; the exfiltration volume per unit area from the bottom and lateral of bioretention were 106.79-396.10 mm/m² and 50.60-147.45 mm/m², respectively; and the runoff reduction rate, runoff peak reduction rate, and runoff delay time of bioretention were 53.46%-96.19%, 18.43%-68.08%, and 288-318 min, respectively. These results suggest that bioretention without an underdrain and with a relatively smaller K_s (saturated permeability coefficient) of in-situ soil might result in longer ponding times and larger overflow volumes. With an increase in K_s of in-situ soil, the underdrain outflow weakens, the exfiltration volume increases, and the runoff control effects improve. Although the groundwater level has little effect on surface ponding, it can cause a stronger underdrain outflow. The shallower groundwater level leads to a larger exfiltration volume when the K_s of in-soil is much smaller than that of the planting layer and leads to a reduced runoff regulation effect for bioretention without an underdrain. Therefore, when locating and designing bioretention systems, the in-situ soil type and groundwater level should be comprehensively considered to ensure that the runoff control target is achieved.