A method for mitigating flooding in existing coastal plain areas includes: A dyke (or dykes) is (are) set/built on seabed outside partial existing coastline of an existing coastal plain area to form an enclosed area; the enclosed area, together with the dyke(s), the reservoir inlet(s) and outlet(s), constitutes a reservoir for containing rainwater; or, part(s) of the enclosed area is (are) of a reclamation area (or reclamation areas). Before the start of forecast heavy rainfall or continuous rainfall, the water level in the reservoir is lowered in advance and the water level of water area for containing rainwater in the existing coastal plain is also pre-lowered, so effective storage capacity for containing rainwater against flooding caused by rainfall in the locality is substantially increased. The present invention is applicable for the flooding control works, reclamation works, roads and highways and their integrated works in existing coastal plain areas.