Based on the indoor rainfall-runoff simulation experiment data from a rainwater wetland system composed of pre-pond, first-order, and second-order wetlands, the effects of design parameters, such as different rainfall recurrence periods, constant water levels, wetland outlet heights, wetland series, and pollution loads on the regulation effect of wetland water quantity and quality were studied. The results show that with the functions of interception, buffering, and storage, the pre-pond wetlands system could regulate the runoff appearance time, volume, and reduction rate, reducing the pollutant concentrations. The stormwater pollution load exported from the watershed was greatly decreased, and there were obvious rules with the change of the design parameters. With an increase in the rainfall recurrence period, constant water level, and a decrease in the height of wetland outlet, wetland series; wetland outflow and duration of outflow increases, and runoff reduction rate decreases. In terms of the event mean concentration (EMC) of pollutants during rainfall, the reduction in suspended solids (SS) in rainwater wetlands is obvious, with a reduction rate of 70%-80%. The reduction effect on the concentrations of other pollutants is not obvious. In terms of the total reduction rate of rainfall pollutants, rainwater wetland has a good interception effect on rainfall runoff pollutants, and with the reduced rainfall recurrence period, increased wetland outlet height, and decreased wetland normal water level, this trend is gradually increasing.