Steel-concrete composite beams, due to their superior mechanical properties, are widely utilized in engineering structures. This study systematically investigates the calculation methods for internal forces and load-bearing capacity of composite beams based on elastic theory, with a focus on the transformed section method and its application under varying neutral axis positions. By deriving the geometric characteristics of the transformed section and incorporating a reduction factor accounting for slip effects, a computational model for sectional stress and ultimate load-bearing capacity is established. The results demonstrate that the slip effect significantly influences the flexural load-bearing capacity of composite beams. The proposed reduction factor, which considers the influence of the steel beam’s top flange thickness, offers higher accuracy compared to traditional methods. These findings provide a theoretical foundation for the design and analysis of composite beams, with significant practical engineering value.