Flexible electronic technology is now widely developed in wearable products for humans and the detection of physiological characteristics, track tracking, and analysis of the living environment for animals and plants. In these application scenarios, flexible electronic devices (FEDs) may work outdoors for a long time and may be exposed to strong solar radiation, which may cause the temperature of FEDs to be too high and affect their stability. Hence, a quantitative description of the thermal properties of FEDs is helpful to provide a reference for the design of related FEDs. This paper establishes a three-dimensional thermal analysis model, verified by the finite element method and experiments, to analyze the temperature increase in the interface of the device under solar radiation and generated by the device itself, and analyzes the parameters that have an influence on the temperature increase in the interface, including the thickness of the substrate and the encapsulated layer and the power of the device. On this basis, the contributions for these parameters to the temperature increase in the interface are further compared.