Abstract: Up to now the natural reserves of Mt.Changbai is the largest inChina,and it is also a component part of the MAB Program.Tian-chiof Mt.Changbain is a boundary lake situated between China and Korea,and it is also the source of the Songhua River.It is very significant tothe environmental protection and the ecology of forest to study thechemical composition of water in Tien-chi Lake.In this study,different techniques and methods have been employed,50 items of the constant and microconstituent elements were determined,the spacial distribution of chemical composition in the lake water wasfound,and the background content of chemical elements of water wascalculated for the first time,and the formational mechanism of thehydrochemistry was clarified.According to investigation and analysis,the content of rare elements,especially rare earth elements as La,Co,etc.,in water of Tian-chi Lake is very high.The consentrations of mostheavy metal such as Cu,Mn,Ni,Zn,Co,etc.,in water approach theClarke values in the hydrosphere.The contents of some heavy metalssuch as Cr,Cd,Hg,Zn,Co,As,etc.,related clos ly to human activityare far lower than the average value of 45 lakes in U·S·A.The content ofPb in water of Tian-chi Lake is particularly high.about 3 times asmuch as the average value of the latter.That is mainly caused by theinfluence of biogeochemical region with higher content of Pb,it hasno relation with environmental pollution.