Low noise amplifier (LNA) is the most sensitive component in the front end of a wireless communication system, which is very vulnerable to coupling attack of high-power electromagnetic pulse (HPEMP). This paper first analyzes the damage mechanism of a high-power electromagnetic pulse to a low noise amplifier, then designs the damage experiment of high-power electromagnetic pulse injection to the low noise amplifier, and verifies the damage mechanism and damage threshold of a certain radar low noise amplifier. The experimental results show that the LNA is not affected by the linear effect of high-power electromagnetic pulse. When the high-power electromagnetic pulse exceeds the damage threshold, thermal melting occurs between the FET grid sources in the device, causing permanent damage to the LNA and performance failure. This conclusion can provide data support for the high-power electromagnetic pulse protection design of this type of LNA.