With the growing demand for wireless communication quality in high-speed railway (HSR), ensuring communication reliability in high-mobility scenarios has become a critical challenge. Constructing a reliable channel model is the key to addressing this issue. To build a highly general and reliable channel model, composite wireless communication channel modeling requires full consideration of the actual operating environment and channel propagation characteristics. With rigorous mathematical modeling and logical reasoning capabilities, the formal method demonstrates significant advantages in complex wireless channel modeling. Focusing on the typical HSR communication scenario of viaducts, this study proposes a high-order logic model of composite wireless communication channels based on a small-scale fading model using the formal method. To address the long-tail characteristic of composite channels, the theorem proving technique is used to verify that the probability density function (PDF) of the composite wireless communication channel conforms to the distribution of the modified Bessel function of the second kind.