Abstract: Under the guidance of China’s “dual carbon” goals, the solar-driven green “ammonia-hydrogen” technological route has become one of the effective solutions for large-scale hydrogen energy applications due to its advantages of carbon-free and high-density hydrogen storage. However, solar ammonia-to-hydrogen reactors still face urgent problems, such as low efficiency and difficulty in scale-up. In view of this, a tubular solar photo-thermal ammonia-to-hydrogen reactor with a coaxial baffle structure adapted to the industrial-scale parabolic trough concentrator is proposed. First, a photo-thermo-fluid-chemical multiphysics coupled model is constructed for the reactor. Then, structure and operation parameter optimizations are conducted for four typical catalyst bed configurations: flat catalyst bed with no baffles (FBNB), flat catalyst bed with inclined baffles (FBIB), coaxial hollowed catalyst bed with no baffles (CHBNB), and coaxial hollowed catalyst bed with annular baffles (CHBAB). The results show that the optimized FBIB reactor achieves the highest systematic solar hydrogen production efficiency (34.91%). The main reason is that the coaxial baffle structure guides the reactants to pass through the catalyst bed multiple times to ensure sufficient reaction, and simultaneously significantly improves the temperature uniformity of the bed. This comprehensive effect is beneficial for promoting the reaction and extending the reactor’s service lifetime. Finally, the temperature field distribution is emphatically analyzed, and the layout structure and parameters of the catalyst bed under the most suitable temperature conditions are obtained. This study can provide theoretical guidance for the development and industrial application of solar photo-thermal ammonia-to-hydrogen reactors.