<p>Petal-shaped fuel rod assemblies are characterized by their compact structure and high power density, making them highly promising for application in small-scale reactors. This study uses the <i>Realizable k–ε</i> turbulence model, <i>Eulerian</i> two-fluid model, and wall boiling model to investigate the spatial evolution of thermal-hydraulic characteristics in 2 × 2 petal-shaped fuel rod assembly under cosine heating conditions. The onset of nucleate boiling (ONB) and critical heat flux (CHF) are determined, and based on these, the flow field, temperature field, void fraction, and heat transfer characteristics in both single-phase and subcooled boiling regions are analyzed. Results show that in the single-phase region, fuel temperature is low and heat transfer coefficient fluctuates periodically at a low level. In the subcooled boiling region, void fraction distribution follows a peak-like pattern along the axial direction due to cosine heating influence. Vapor phase generation leads to the increased fluctuation amplitude of coolant transverse flow, evaporation heat flux gradually dominates, leading to a significant increase in the heat transfer capacity of the rod bundle channel. CHF occurs at the inner concave arc of the fuel rod slightly above the channel center, and void fraction peaks at <i>Z</i>_CHF; moreover, the deterioration of heat transfer causes a significant decrease in heat transfer coefficient.</p>