For　quantitative　study　of　time　constant　and　degradation　ratio　of　degradation　parameters　which　correspond　to　different　failure　mechanisms　in　pseudomorphic　high　electron　mobility　transistor　(PHEMT)　gate　current　degradation　process,　a　PHEMT　gate　current　degradation　model　is　established　based　on　the　relationship　between　reaction　volume　concentration　and　reaction　rate　in　the　process　of　degradation.　The　degradation　law　of　PHEMT　electrical　parameters　is　obtained　using　online　experiment　method.　The　parameter　degradation　law　with　the　time　is　analyzed　and　the　failure　mechanism　which　affects　gate　current　degradation　in　different　time　period　is　obtained.　Meanwhile,　based　on　the　gate　current　parameter　degradation　model,　time　constant　and　degradation　ratio　of　degradation　parameters,　which　correspond　to　different　failure　mechanisms,　are　also　obtained.