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.