Abstract: Based on the definition of the ocean environment, the calculation methods of additional force in heaving and rolling motion are analyzed, and the modified methods of momentum equations and boundary conditions are obtained. Based on these, the sub-channel code ATHAS, which is with self-owned intellectual property, is modified and a sub-channel code named ATHAS/OE is obtained, which can be applied to the ocean environment. Thermal-hydraulics analysis of MUTSU reactor in heaving and rolling motion is carried using ATHAS/OE. The results indicate that in heaving motion, the calculation results of ATHAS/OE and the modified COBRA-IV match well. The outlet mass flux and quality of sub-channels and MDNBR of reactor are oscillated periodically. Because of the modification of Critical heat flux（CHF）calculation method in heaving motion, the MDNBR of reactor in heaving motion always decreases compared with that in land-based condition. The MDNBR of reactor is oscillated slightly because of no modification of CHF calculation in rolling motion.