Electronic system and device are vulnerable under intensive electromagnetic pulse (EMP) environment, where low noise amplifer (LNA) is a typical sensitive instance for electromagnetic energy. This work focuses on the EMP-induced damage effect of GaAs pseudomorphic high electron mobility transistor (PHEMT), which is the core part of LNA. Using the simulation softeware Sentaurus TCAD, an EMP-induced damage model of the GaAs PHEMT is established in this paper, and verified through the experimental result. It is shown that the damage position of the device under the injection EMP exists in the center area under gate terminal. Based on this model and aiming at EMP parameters and external resistances, the influence of the external conditions on the damage effect of the device is investigated. The results indicate that the damage time is related to EMP parameters obviously:1) the damage time is inversely proportional to EMP amplitude since higher power density is absorbed under a stronger EMP; 2) the damage time is in direct proportion to signal rising time since the breakdown time is postponed under EMP with a slower rising edge. Furthermore, it is found that a load resistor is able to weaken current channel which is effective in delaying the damage process, and this effect is more obvious, with load resistor connected with source terminal. It should be noted that the results are beneficial to and valuable in hardening method against EMP of semiconductor devices. It is feasible to design external circuit protection units, aiming at attenuating signal amplitude and increasing the rising time of injected pulse. Another effectual approach is to enlarge the source series resistance under the premise of the performance meeting the requirements.