Soft bellows robots exhibit exceptional movement capabilities owing to their considerable retractability and high flexibility. Nonetheless, accurately describing the relationship between their actual movements and the forces they exert remains a significant challenge. To address this challenge, this paper proposes a dynamic modeling method for soft bellows robots. Initially, a soft bellows robot is designed as the subject of study, and its movement is categorized into two modes: straight crawling and steering, based on their respective functions. Subsequently, leveraging the constant curvature bending assumption, the relationships between the deformation and pose of the soft bellows actuators (SBAs) constituting the soft bellows robot are meticulously analyzed. Dynamic models for the SBAs are then established. Finally, the validity of these dynamic models for the SBAs is confirmed through controlled testing, thereby verifying the two movement modes of the soft bellows robot. In summary, the dynamic modeling method presented in this paper enhances the theoretical framework of soft bellows robots and provides crucial theoretical support for precise control.