Rainwater systems, which collect rainfall from the roof surface, are an attractive alternative technology, and one of the cleanest water resources. Microorganisms can be treated by disinfection processes or by inducing a biofilm, but particulate matter can only be easily removed with effective storage tank design parameters. In this study, therefore, the effects of rainwater tank design parameters on the treatment of particulate contaminants are analyzed. The J-type inlet pipe seems to maintain stable rainwater quality by suppressing the resuspension of bottom sediments during rainfall inflow. The intermediate wall can prevent sediment from spreading to the whole storage tank, and provide functions such as securing the stable water quality of the outflow area and managing sediment flow to the inflow region. The baffles can simplify tank management such as sediment drainage and tank cleaning by leading to more sedimentation of particles in the unit area. Based on the water quality monitoring results of pH, BOD, SS, and turbidity at the field scale, it is possible to supply a certain quality of water by optimizing tank design parameters alone even if particulate contaminants flow into the rainwater tank.