Abstract: Based on the heat load method,an analysis is performed for the removing ability of decay residual heat of core and spent fuel pool of core decay heat recovery circuit（JNA）、spent fuel pool cooling system（FAK） of The Third and Fourth Unit in Tianwan Nuclear Power Plant after the modification of fuel assembly.The results show that the removing ability of decay residual heat of JNA system of The Third and Fourth Unit in Tianwan Nuclear Power Plant could satisfy the need of removing of decay residual heat under normal condition,anticipated operating condition and accident condition.It is also pointed out that the ability of FAK system can satisfy the need of removing of decay residual heat of spent fuel pool.