Under the impact of high-power electromagnetic pulses, RF integrated microsystems are prone to generate load mismatch, which may lead to system failure or even damage. Based on a real-time waveform test method, this paper analyzes the mechanism of load mismatch of RF devices leading to device damage. This method utilizes vector network analyzer as the main instrument, obtaining the real-time voltage and current waveforms with reflection signal and phase reference module. Active load-pull technique is used to simulate high power coupled electromagnetic pulse injection and ruggedness test under 39∶1 VSWR is done. Furthermore, harmonic source injection is newly applied to simulate the electromagnetic interference caused by harmonics, obtaining the harmonic impedance mismatch characteristics of the device. The test results of actual heterojunction bipolar transistor device indicate that the combination of fundamental and harmonic interference components causes the transient peak value of the output voltage to be higher, which is more likely to damage the device. Hence the fundamental and harmonic components should be considered when conducting electromagnetic protection.