This paper proposes a novel co-simulation method for analyzing the interaction between circuit performance and electro-thermal multiphysics effects in a radio frequency (RF) power amplifier at the PCB board level, particularly under the influence of an external electromagnetic pulse (EMP). First, a SPICE equivalent model of the power amplifier circuit is developed, and the RF output node voltage is calculated. These results are subsequently used as boundary conditions in the electro-thermal analysis. The temperature distribution is obtained using electro-thermal multiphysics coupling theory. Additionally, since both circuit components and the PCB board are typically temperature-sensitive, a reverse analysis is conducted to study the effect of temperature variations on circuit node voltage. Numerical examples demonstrate the accuracy and efficiency of the proposed method, with a comparison to COMSOL Multiphysics® validating its performance. Furthermore, the method is successfully applied to analyze the electromagnetic interference (EMI) in an RF power amplifier. The results indicate that when the amplitude of the external pulse signal exceeds 1 V, the amplifier temperature can increase to 555 K, leading to high-temperature distortion in the power amplifier.