After subjecting to different high-field stress and gate stress，a recoverable degradation has been found，consisting of the decrease of saturation drain current IDsat and maximal transconductance gm，and the positive shift of threshold voltage VTH. AlGaN/GaN HEMTs were degradated more with the high-field strss voltage and the stress time increasing. Relatively，under gate pulse stress and on-state gate stress，the degradation was more obvious than under off-state gate stress. By analyzing the shift of primary parameters，we found that the hot carriers generated by impact ionization and trapped by traps in AlGaN barrier layer，and the emission of electrons from gate electrode filling in surface states at high gate-to-drian electric fileds, were the primary reasons causing device degradation after different stress. Off-state stress，on-state stress and pulse-state stress measurements reveal that the passivation treatment just changes short-time current collapse into long-time degradation，which dose not solve the reliability problem of AlGaN/GaN.