Abstract: The tube bundle structure of AP1000’s passive residual heat removal heat exchanger (PRHR-HX) was improved base on the CFX software and the influence of tube bundle structure on the performance of PRHR-HX was also analyzed. The results show that in the region of single-phase natural convection and mild boiling, the heat transfer coefficient of triangle bundle arrangement is widely superior to the existing design and the capability increases with the decrease of pitch size. To ensure the heat transfer performance in the boiling zone, the pitch size should not be less than 1.5d_o, so that the bubbles can easily escape from the heating surface.