Flash floods are the important events of the hydrological regime of rivers in arid areas.In the Tarim River,northwestern China,flash floods are being monitored.The observed data and investigation demonstrate the difference in time,place,frequency and intensity of their occurrences.Two main flash floods are put forward,they are rainstorm flash flood (RFF) and glacier lake outburst flood (GLOF) by two cases of flash flood in the two tributaries of the Tarim river presented in this paper.It analyses and compares the causes and the development of the two kinds of flash floods.Through further discussion about influence of flash floods on the main channel of the Tarim River,conclusion can be drawn that the greatest flood in record of the main channel come from the GLOF of the upper reaches of the Kunmalik River,especially augmented by great ablation flood.Finally the advantages and disadvantages from flash floods to the environment of the catchment are demonstrated in the paper.