An analytical large signal model based on semi-empirical formulas is presented for describing the DC characterization of GaAs Pseudomorphic High Electron Mobility Transistor (pHEMT), which included pad capacitances, intrinsic elements and extrinsic elements is in good agreement with the theoretical results obtained by performing device simulation. The accurate temperature rise in the GaAs pHEMT's 2-dimensional electron gas (2DEG) channel under self-heating effect is analyzed by multi-scale multiple heat-sources thermal analysis. And the change of the model's physical parameters dependent on temperature is discussed. Then the degradation of pHEMT's DC characteristic at different gate voltage is predicted by analyzing the change of the electrical parameters dependent on temperature. By this way, the effective fault injection for the Prognostics and Health Management (PHM) system of active phased array radar can be achieved.