Abstract: The demand for laser communication and data transmission technology between atmosphere and ocean is rapidly increasing, necessitating robust and reliable air-sea cross-domain communication technologies. By deriving analytical formulas for the wave structure function, spatial coherence length, and spatial coherence function, this work analyzes the downlink transmission of plane wave and spherical wave two-wave model for air-sea cross-domain mixed channel using the Rytov approximation method. Numerical simulations are conducted to validate these findings. This work reveals the coherence characteristics of two-wave model beams during transmission through complex and weak turbulent media in the atmosphere and ocean. It is found that the Kolmogorov five-thirds power law of wave structure functions in atmospheric turbulence is adaptable to the inertial range of oceanic turbulence. Furthermore, this work explores the variations of wave structure function, spatial coherence length, and spatial coherence function for plane wave and spherical wave during transmission through atmospheric and oceanic turbulence. This work provides important theoretical guidance and technical support for coherent detection of the robustness and reliability of laser transmission for air-sea cross-domain communication.