The heat transfer performance of a high-density polyethylene helical coil heat exchanger (HCHE) which is adopted by a seawater-source heat pump system (SWHP) is experimentally and numerically studied. The effects of seawater flow rate on heat transfer performance of the HCHE are investigated and the correlation between Nusselt and Reynolds number is fitted by the experimental results of cooling condition. The fitted results are compared with other similar studies of helical heat exchangers. Mathematical models in cooling condition, non-icy condition and icy condition are developed with the calculated external convective heat transfer coefficient. The simulation results are verified by the experimental data of icy condition and they show good agreement with each other. The effects of different key parameters on the heat transfer performance of the HCHE are investigated, such as inlet temperature, velocity of intermediate medium, pipe length and diameter, temperature of seawater and icing outside the pipes. The results indicates that the consideration of seawater flow rate in the study of subsea heat exchangers is much closer to the reality and the development of the mathematical models is very helpful in the designing and optimizing of the subsea heat exchangers used for the SWHP system. (C) 2016 Elsevier Ltd. All rights reserved.