In　this　paper,　the　linear　relationship　of　forward　Schottky　junction　voltage　of　AlGaN/GaN　HEMT　with　temperature　was　investigated.　It　was　used　as　temperature　sensitive　parameter　to　determine　the　channel　temperature　at　its　normal　working　state　by　fast　switch　circuit　technique.　The　regress　of　tested　data　was　used　to　reduce　the　scatter　error　too.　The　thermal　resistance　constitution　was　extracted　from　the　transient　heating　response　curves.