To　explore　degradation　law　and　failure　mechanisms　of　AlGaN/GaN　HEMT　under　different　temperature　conditions,　the　step-temperature　stress　experiments　are　carried　out.　The　results　show　that　the　drain-source　current　decreases　with　the　aging　time　when　the　junction　temperature　range　from　139　degrees　to　200　degrees;　while　the　drain-source　current　increases　with　the　aging　time　when　the　junction　temperature　range　from　200　degrees　to　352　degrees.　When　the　junction　temperature　is　less　than　200　degrees,　the　AlGaN　ionized　donor　atoms　cause　the　Schottky　barrier　height　of　the　AlGaN/GaN　HEMT　device　to　rise;　and　when　the　junction　temperature　is　higher　than　200　degrees,　the　diffusion　of　impurities　oxygen　in　surface　causes　the　Schottky　barrier　height　of　the　AlGaN/GaN　HEMT　device　to　fall.　The　barrier　height　can　affect　the　threshold　voltage　which　leads　to　the　change　of　drain-source　current.　Therefore,　the　degradation　of　the　drain-source　current　is　caused　by　the　change　of　the　Schottky　barrier　height.