Based on the analysis of incoming water and sediment of main hydrological observation and typical reach and riverbed boundary condition in middle-lower reaches of the Yarkant river, and the analysis of three major factors which have influence on alluvial river riverbed evolution, the paper has analyzed the cause of formation of wandering type river in plain areas of the Yarkant river. The preliminary conclusion shows that the main cause is riverbed boundary condition and the subsidiary condition is incoming water and sediment. According to the above results, it is suggested that both flood prevention and bank protection in middle-lower reaches and large-scale hub control engineering in the upper reaches should be constructed when it comes to basin management.