Abstract: A large number of heat exchangers are used in the passive safety system of nuclear power plants. Under the condition of natural circulation, the flow of the fluid in the heat exchanger is weak, that may be in laminar flow and leads to the low heat transfer coefficient. In order to enhance the heat transfer, the new twisted tape named coaxial cross triple twisted tape is presented. The heat transfer effect of the water in the tube fitted CCTTT with different twist ratio (y = 2,3,4,∞) is studied by numerical simulation. When the Reynolds number changes from 40 to 1200, the results show that CCTTT can effectively enhance the heat transfer of single phase laminar flow in the tubes. Moreover, with the decrease of twist ratio, the heat transfer effect and the performance evaluation criteria (PEC) increased. The PEC number of CCTTT with twist ratio of 4.0 is 2.02, while that of CCTTT with twist ratio of 2.0 is 2.64. Compared with other working fluids, at the same twist ratio, the PEC number of CCTTT increases with the improve of Prandtl number.