Abstract: Steam generators are critical core components in lead-bismuth fast reactor (LFR), playing an irreplaceable role in ensuring the safe, economical, and reliable long-term operation of the entire reactor system. As key equipment for heat exchange between the primary and secondary circuits, they directly affect the reactor’s thermal efficiency, power output stability, and overall safety margin, as they transfer the heat from nuclear fission in the primary circuit to the secondary circuit to produce high-pressure steam for power generation. Among various steam generators, helical coil once-through steam generators (HCOTSG) stand out due to their compact structure, high heat transfer efficiency, and strong adaptability to complex environments. Compared with traditional U-tube and straight-tube steam generators, HCOTSG have unique advantages in ship or offshore platform reactors, especially in space utilization, thermal stress resistance, and adaptability to dynamic conditions—their compactness is crucial for marine applications with limited space, and high heat transfer efficiency ensures effective heat transfer to improve overall power efficiency. To investigate the effects of rolling and tilt from complex ocean conditions on HCOTSG’s thermal-hydraulic performance, a high-precision thermal-hydraulic analysis code was established based on fluid mechanics and heat transfer principles. Ocean conditions’ influence on HCOTSG’s internal flow and heat transfer was incorporated by modifying the momentum equation, considering additional inertial forces and gravitational components from motions. The model’s reliability was verified using existing experimental data and literature, ensuring it could simulate HCOTSG’s thermal-hydraulic characteristics under normal and abnormal conditions. Based on model validation, the code was used for detailed transient calculations of LFR under typical ocean conditions. Key operating parameters were explored, including heaving motion periods, heaving motion amplitudes, heaving-tilt coupling directions, and angles. Key thermal-hydraulic parameters were monitored to analyze their variation rules. The results show that HCOTSG’s thermal-hydraulic parameters are closely related to heaving motion period and amplitude. With increasing heaving motion period, each parameter’s variation period increases while its amplitude decreases, due to reduced inertial force impact. A larger heaving motion amplitude causes more significant parameter fluctuations, as it leads to greater changes in HCOTSG’s internal flow field and heat transfer. Under heaving-tilt coupling, tilt direction significantly affects system safety. Tilt along the x-axis results in the maximum secondary-side outlet pressure with minimal heat transfer increase, imposing the most prominent impact. With increasing tilt angle, secondary-side outlet pressure rises, heat transfer decreases, and primary-side outlet temperature increases, but the overall influence is smaller than that of heaving motion period and amplitude. The findings show that thermal-hydraulic parameter fluctuation amplitude decreases with rising heaving motion period and increases with growing heaving motion amplitude; Tilt angle has a weak effect under coupling conditions. These results provide theoretical support and technical reference for HCOTSG’s design optimization, safety assessment, and operation control in marine LFR, improving the reactor’s adaptability under complex ocean conditions.