In this paper, the damage process and mechanism of the typical high electron mobility transistor by injecting high power microwave signals are studied by simulation and experiment methods. By using the device simulator software Sentaurus-TCAD, a typical two-dimensional electro-thermal model of high electron mobility transistor is established with considering the high-field saturation mobility, Shockley-Read-Hall generation-recombination and avalanche breakdown. The simulation is carried out by injecting the 14.9 GHz, 20 V equivalent voltage signals into the gate electrode. Then, the distributions of the space charge density, electric field, current density and temperature with time are analyzed. During the positive half cycle, a conduction channel appears beneath the gate electrode near the source side within device. It is found that the electric field is extremely strong and the current density is very large. Therefore, the temperature increases mainly occurs beneath the gate electrode near the source side. During the negative half cycle, because of the concentration of the large number of carriers induced by avalanche breakdown, the electric field is stronger than that in the positive half cycle. But the current density is lower than that in positive half cycle. Therefore, the increase of temperature is dominated by the electric field. With the effects of both strong electric field and high current density, the temperature of the transistor rises in the whole signal cycle. In addition, temperature in the positive half-cycle rises faster than that in the negative half-cycle.Furthermore, the peak temperature appears at the location beneath gate electrode near the source side because the electric field and current density are strongest in this area. When the temperature within the device is higher than 750 K, intrinsic breakdown occurs in GaAs material, so the heating process becomes quicker. With the temperature increases, the GaAs reaches its melting point, and the device fails permanently. Furthermore, taking the original phase of 0 and for example, we discuss the influences of different original phases on damage process. It is shown that when original phase is zero, the temperature increase rate is faster, and the burn-out time is shorter.Failure analysis of high electron mobility transistor devices damaged by microwaves is carried out with scanning electron microscope, and the simulation results are well consistent with the experimental results. The conclusion may provide guidance for studying high power microwave defense of low noise amplifier and rugged design of high electron mobility transistor in fabrication technology.