Recently,with the fast development of the important regional cooperati on actions in e,conomy such as the economic cooperation of Lancang,Mekong River Subregion,the changes,regularity and influence of the water resource system in t his drainage basin have become the focus of the attention and hotspot of researc h internationally.In order to investigate the basic facts which the climate chan ges influence on the hydrological and water ecological environment variations of the Lancang,Mekong River Basin,this paper studies the multi,timescale characte ,ristics of the transboundary flow variations for the Lancang River and its resp onse to the changes of precipitation field over Yunnan based on the observationa l data of the Lancang River flow and precipitation field in Yunnan and using the wavelet transform and correlation analysis method. The results show that the an nual variations for the Lancang River flow in the upper,lower reaches obviously appear multi,timescale features,their difference are mainly on the smaller times ,cales,but they are very analogous on the larger timescales.There are very signi ficant correlations between the annual variations of the Lancang River flow in t he upper,lower reaches and the precipitation field over Yunnan.The multi,timesca le characteristics of the annual precipitation over Yunnan are analogous to the ones of the Lancang River flow in the upper,lower reaches.In brief,the main reas ons for the transboundary flow variations for the Lancang River are the changes of the precipitation field over Yunnan,specially on the larger timescales,the fl ow changes of the Lancang River in the upper,lower reaches have highly response features to the variations of the precipitation field over Yunnan.