This paper addresses the structural adaptability and dynamic stability challenges faced by unmanned underwater-air cross-media vehicles during the transition between gas and liquid media. To overcome these issues, a coaxial dual-pitch configuration scheme is proposed. Numerical calculations are carried out using the fluid dynamics software STAR-CCM+, and the VOF multiphase flow model and DFBI six-degree-of-freedom motion coupling method are adopted to establish a numerical simulation system for the cross-media motion of the vehicle. The fluid dynamic responses under different submersion and emergence conditions are analyzed. The study shows that during the submersion process, an 80° inclination significantly reduces the impact load by 42%, and the peak axial force is positively correlated with the square of the velocity; at a 70° inclination, by enhancing the flow symmetry, the steady-state fluctuation energy of the vertical force is reduced by 60%. The dynamic characteristics of the emergence process indicate that a 70° inclination combined with a speed parameter of 0.3–0.4 m/s can balance the axial resistance and motion stability, and the amplitude of the free liquid surface disturbance is significantly reduced compared to high-speed conditions.