Abstract: Flood is the prime natural disaster in the Taihu Lake drainage area. Based on the data of climate, relief, human activity and disaster records, this article mainly deals with the factors which control the formation and evolution of flood.The formative factors include rainfall and relief. The peak rainfall can reach to 203 mm/h, 582 mm/d and 2356 mm/a at individual station. Three-fourths of the drainage area is plain, half of which is below high tide level. Permanent waters (lakes, rivers and ponds) and paddy fields together cover 55% of the drainnage area (36500 km²) in summer, the main flood season. The gradient of water surface in the major discharge waterway, the Huangpu River, is 6.5?10^-6 in average and is even negative at high tide.The frequency of flood has been increasing in the past two thousand years. The major causes are:(1) the crust of this area has been sinking at the rate of 1.2 mm/a; (2) the coastline of the Changjiang River delta has been continuously moving toward the East China Sea; (3) as a result of (1) and (2), the gradient of the discharge waterway has remarkably decreased to 1/3; (4) the population density has increased from 6/km² to 900/km².In the 21st century, the rainfall of the drainage will increase by 40%-50% due to the global warming climate, the sea level of the Changjiang Estuary will rise about 40 cm-60 cm. Consequently, the threat of flood will be aggravated in the next century.