For general tree-distributed river network,the flow at the upper stream is always flowed to the lower stream.The flow at the down section is always converged from the upper reach section.However,in the area of high dense flat river network,especially,in the area with a lot of man-made controlling hydraulic engineering structures and the influence of tidal flow(such as Taihu Basin),it is very difficult to determine where the water or section flow converges from,because of its uncertain flow direction.It will be very interested in practical ends about such problems.For example,as diverted water from Yangtze River to Wangyuhe River by Changshu hydraulic engineering junction,where does the water flow to and how much diversion water in the each section of river network fraction or each reach is diverted from Changshu junction?There is significant sense in practical ends on flat river network fraction simulation method.This paper mainly introduces the flat river network fraction method and its implement in Taihu Basin.