This article focused on the serious flood in the middle and lower reaches of the Yangtze River in the year 1849. The author consulted a variety of resources about this flood， such as archives， local gazetteers， diaries and other personal literatures， and based on the countylevel index and details of disaster， spatial and temporal distribution of the serious flood disaster is reconstructed. It was shown that a westtoeast zonal distribution of the disaster was between N28°and N33°， with the extremely serious band along N31°. Continuous rainfall was from May 18th to July 18th， with three heavy rains events each lasting more than 10 days during this process. This article pointed out that， this particular disaster covering the whole Yangtze drainage was behaved as a waterlog rather than a flood. The flood is very different from other instrumentrecorded alloverbasin flood that the rainfall was concentrated in the Tai Lake basin in eastern part of the Yangtze River. The main cause of these features in this disaster was the occupation of the plum rain season which had lasted 62 days and the precipitation obviously increased. It should be also considered that the summer monsoon was weaker in that year， and the Subtropical High Ridge Line was unusually bended southwards.Furthermore，the branch of westerly wind played a strong role together with the development of the Meridional circulation， so that it could be deduced that the branch of westerly wind was abnormally located southwards. Besides， the irregular activities of cold air in summer were probably the real factor which drove the rain band staying along the riverside of the Yangtze River for such a long period of time.